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Stress Analysis of Centrifugal Compressor Connected Piping Systems using Caesar II

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

Centrifugal compressors are considered to be one of the most critical pieces of equipment in the piping industry. For increasing the pressure of gaseous fluids centrifugal compressors are widely used in refineries and petrochemical complexes. From a stress analysis point of view, the compressor systems are critical because:

  • Equipment being rotary is prone to vibration.
  • The allowable nozzle loads are very less. Normally combined suction and discharge nozzle analysis is required to be performed.
  • As the pressure increases due to compression the pipe thickness increases which increases the rigidity of the connected pipe and automatically the loads at the nozzle increase.

However, the good part is that normally compressor connected pipes do not have much temperature.

In this article, I will try to explain the procedure to analyze the method followed to analyze such systems.

centrifugal compressor
Fig.1: Typical Piping System Connected to Centrifugal Compressor

Inputs required for Compressor Piping Stress Analysis

The following documents are required while analyzing a compressor-connected piping system.

  • Process P&ID.
  • Line List for line parameters.
  • Pipe Material Specification.
  • Equipment Vendor GA drawing.
  • Suction and discharge nozzle displacements in proper axes with direction.
  • Line isometrics.
  • Allowable nozzle loads.

Building the Compressor piping system in Caesar II

In Fig. 1, a typical piping model connected to a centrifugal compressor and scrubber is shown. If we have to analyze such systems then the following steps have to follow:

  • Model the pipe from isometrics in a similar way as you normally do for the other systems. Use Line parameters from PMS and Line List. If the line is insulated then take the exact density of the insulating material as that value will impact the analysis. If the fluid density is available then use it otherwise leave it blank (zero density as usually gas). A Caesar plot of the above system is shown below for your reference.
typical suction piping model
Fig. 2: Caesar Plot of the system shown in Fig. 1
  • It is very difficult to model the compressor using GA drawings and get the actual thermal displacements at the nozzle points. So the vendor provides the thermal displacement values at operating temperature. So now find the compressor nozzle displacements from GA drawing or mail communications and input the same at the compressor nozzle node as shown in Fig 3.
Caesar Spreadsheet Explaining the input of nozzle displacements
Fig. 3: Caesar Spreadsheet Explaining the input of nozzle displacements.
  • Model the scrubbers taking data from Vendor GA.
  • Now prepare all the load cases and run the system statically to check the results. Make the system stresses within the code’s allowable limit. Make scrubber nozzles within vendor allowable. Refer to the next paragraphs for compressor nozzle qualification.

In general practice, compressor nozzles are to be qualified by API 617 or NEMA SM23. There are three separate checks to ensure the proper working of the compressor.

  1. Individual component forces (Fx, Fy, Fz and Mx, My, Mz, each component at each nozzle) at the nozzle point must be within the limit specified in the code. (For equations refer to annex 2E of API 617)
  2. The resultant force and resultant moment of each nozzle (suction and discharge separately) must be within the limit specified in the code. (For equations refer to annex 2E of API 617)
  3. The combined resultants of the forces and moments of the inlet, sidestream, and discharge connections resolved at the centerlines of the largest connection (resolution point) should not exceed the limit specified in the code. (For equations refer to annex 2E of API 617)

Always remember that the Caesar axis and API axis may not be the same. So you need to convert the allowable forces into the proper Caesar axis and then compare. The API 617 axis system is reproduced in Fig. 4 for your reference.

co-ordinate system
Fig. 4: Co-ordinate system as per API 617 for nozzle force and moments.

Sometimes vendor allows using some factor. Always check with the vendor at an early stage of the project and agree on this.

The resolution point is the point with respect to which all forces and moments of suction and discharge nozzles will be resolved for a combined study. As per the code, it is the flange face of the largest compressor nozzle (normally the suction nozzle). Measure the position (dimensions) of the other nozzle with respect to this point for a combined study. A typical diagram showing suction and discharge nozzles from a GA drawing is reproduced in Fig. 5 for your understanding.

Compressor GA drawing
Fig. 5: Typical GA drawing showing suction and discharge nozzle.

Qualification of Compressor Nozzles

From the above paragraph, it is clear that to qualify the compressor nozzles you need to model and analyze the discharge nozzle and connected piping system (discharge side piping is shown in Fig. 9). So follow the same procedure as mentioned for the suction nozzle and analyze.

Compressor nozzle loads are checked in the NEMA SM23 module of Caesar. To perform the same click on Analysis and then NEMA SM23 as shown in Fig 6.

NEMA SM 23 module in Caesar II
Fig. 6: Opening NEMA SM 23 module in Caesar II

The module will open the spreadsheet (Shown in Fig. 7) where you have to enter the parameters of suction and discharge nozzles, loads at suction and discharge nozzle, and distance from the resolution point. Enter the factor of allowable increase if the vendor allows doing so. Normally as per API 617, the value is 1.85

NEMA SM 23 spreadsheet
Fig. 7: NEMA SM 23 spreadsheet in Caesar II

If you provide a node number Caesar will automatically select the loads from your static analysis upon clicking the proper load case.

After loads are provided for both the suction and discharge nozzle click on the run button to see the analysis results. It will show the results as passed or failed. If failed check the reason for failure and reduce the same force or moment value to get it qualified. A typical output report is shown in Fig. 8 for your reference.

NEMA output
Fig.8: Typical Caesar Output of NEMA SM 23 module

Additional considerations for Compressor Piping Analysis

  • Perform alignment check (Anchor free analysis) in a separate file and keep nozzle movements within the limit as specified in API RP 686.
  • In most cases supports near compressor suction and discharge nozzles are Spring hangers. So, Create an additional load case as WNC+H- (SUS) type and qualify the nozzles in that case too.
  • Normally Compressor nozzles are qualified in operating temperature (and sustained) only. However, if the client asks you have to do so in the design temperature case.
  • If variable springs are used to qualify Compressor nozzles to keep the variability as low as possible (less than 10%).
  • Normally modal frequency analysis is not required for centrifugal compressor-connected systems.
  • In general, Compressor nozzles are not required to qualify in occasional load cases.
  • Don’t forget to request nozzle displacements at the early stage of the project.
Compressor Outlet
Fig. 9: Compressor Discharge side piping of a typical system

Few more useful resources for you…

Articles related to Compressor
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Piping Design and Layout Basics
Piping Stress Analysis Basics
Piping Materials Basics
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Flange Leakage Evaluation based on NC 3658.3 Method method using Caesar II

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

Flange leakage checking is a very important activity in pipe stress analysis. Flange leakage checking ensures that there will be reduced chances of flange leakage. There are various methods available for flange leakage checking in conventional stress analysis software packages like Caesar II. NC 3658.3 method is one such popular flange leakage checking method. In this article, we will discuss the step-by-step methods for performing flange leakage analysis using the NC 3658.3 method in the software Caesar II.

NC 3658.3 provides a flange leakage evaluation method based on the NC code for ASME B16.5 flanges using high-strength bolts. Caesar II software has a nice module in its input spreadsheet to use this method efficiently and with ease. In the following paragraphs, I will describe the methods of flange leakage evaluation by NC 3658.3 method using Caesar II.  

Criteria for NC 3658.3 Method Application

NC 3658.3 Method for flange leakage evaluation can only be applied if the following two conditions are met:  

  1. Flanges, bolts, and Gaskets used are designed based on rules as specified in ASME B 16.5 and
  2. Bolting material must have an allowable stress value at 100°F(38°C) >=20000 psi (138 MPa) (High Strength Bolting)

Governing Equations as Per NC 3658.3 method

By NC 3658.3 method the generated external moment (Mfs) is limited to a value as provided by the below-mentioned equation:  

Mfs<=3125(Sy/36000)C X Ab     (in U.S. Unit);

Here

  • Sy=Yield strength of flange material at the temperature
  • Ab= Bolt area
  • C=Bolt Circle Diameter
  • The ratio of Sy/36000 has to be taken less than unity.  

So it is obvious from the above equation that we have to enter the bolt area, bolt circle diameter, and flange material yield strength as input into Caesar II Spreadsheet for flange evaluation using the NC method.  

Flange Evaluation steps followed in Caesar II

  • Select Flange Node (From/To/Both) and Calculation Type (NC-3658.3) as shown in Fig. 1.
  • Input Bolt circle diameter from ASME B 16.5.
  • Input Yield Strengths at the temperature from ASME BPVC code Section II Part D Table Y1.
  • Calculate the Bolt Area (Ab) as shown below and input it in the required place:

Bolt area, Ab is the total cross-sectional area of the bolts at the root of the thread or section of least diameter under stress.  

Bolt Area Calculation for NC method

Calculate the bolt area as mentioned below

Droot = D1bsc – 2*0.21650635P & D1bsc  = Dbsc – 2*h

Where

  • Droot = Root diameter of the bolt
  • Dbsc = Basic major (nominal) diameter of bolt (as per ASME B1.1, para 8.3)
  • D1bsc = Basic Minor diameter (as per ASME B1.1, para 8.3)
  • h = Height of thread (as per ASME B1.1, Table 5)
  • P = Pitch of Bolt (as per ASME B1.1, Table 1)  

Root Area of bolt (Ab) = n x [(pi/4)* Droot2)]                            

Where n = No. of bolts as per flange pressure rating class

Alternately, The bolt area can be directly taken from ASME PCC-1, Table H-1/H-1M for inch series and metric threads respectively.

Input Spreadsheet from Caesar II for flange leakage check by NC 3658.3 method
Fig.1: Input Spreadsheet from Caesar II for flange leakage check by NC 3658.3 method

Now go to the Load Case Option Editor (Fig. 2), select flange analysis temperature based on operating/design conditions, and click the Run button for analysis.

Select the Load Case(s) for which Flange Leakage Check has to be performed and see the results. If the ratio of Flange Stresses to Allowable Stress is less than 100 % (See Fig. 2), Then the flange is within the allowable limit and should not produce leakage. If it is more than 100%, Then try to reduce moments at the flange by re-analyzing the system and then re-check using the above method.

A typical Caesar II flange leakage analysis report
Fig.2: A typical Caesar II flange leakage analysis report

Allowable Stress for NC 3658.3 method as per Caesar II

Caesar II Allows the user to check the NC flange leakage check at 9 temperature case conditions (operating condition ), Hydrotest, and Sustained condition at a time.

In CAESAR II, an Equivalent flange Stress S is calculated based on the following formulas, and then the calculated stress is compared to Sy (or 2*Sy for occasional load cases), in the following manner:

S = 36,000* Mfs / (CAb * 3125) ≤ Min(Sy, 36000) (non-Occ)

S = 36,000 * Mfd / (CAb * 3125) ≤ 2.0 * Min(Sy, 36000) (Occ)

Note that both the above equation is in the FPS unit. For metric or other unit systems, the Caesar II software converts the 36,000 values in the above equations to the appropriate set of units.

Is the NC 3658.3 method applicable for ASME B16.47 Flanges?

The specific clause of NC3658.3 clearly states that the method is only applicable for ASME B16.5 flanges. So, in general, the method is applicable for flanges up to 24″ sizes. However, piping engineers frequently encounter piping systems having sizes greater than 24 inches means flanges from the ASME B16.47 standard. Several studies have already been done regarding the applicability of the NC 3658.3 flange leakage checking method for ASME B16.47 flanges.

One such published paper with the title “Use of NC 3658 flange design rules for ASME B16.47 flanges- An analytical and finite element based study” by Anindya Bhattacharya & Michael P Cross concludes that the simple method of checking a flanged joint based on NC 3658.3 can be extended to B16.47 flanges, but there are some exceptions as mentioned below:

  • NC 3658 flange design rules are not applicable for flanges of size 60 inches 600 Lb.
  • Also, for sizes >=36 inches, 600 Lb. and higher flange ratings, as the operating stress on bolts is higher than twice the material allowable stress, the NC 3658 rules are not applicable.

For more details, you can refer to the paper from the following site: https://asmedigitalcollection.asme.org/ESDA/proceedings-abstract/ESDA2014/45837/V001T05A009/232054

Free Trial Version of Pipe Stress Analysis Software and Online Pipe Stress Analysis Training Course with Certificate

Written by Alex Matveev in Caesar II,PDMS-E3D,Piping Stress Analysis,Piping Stress Basics,Start-Prof

The PASS/START-PROF is a pipe stress analysis software that has a long history since 1965, see update history. It is industry standard in several countries for power, process, oil and gas pipelines, and district heating networks, used by 3000+ companies.

To learn what is PASS/START-PROF pipe stress analysis software you may visit the resource center. You will find a webinar recordings, articles and presentations in pdf. Also you may refer to online help system.

Enroll the complete 25 hours Pipe Stress Analysis Training Course! You will get a free 30-day access to the pipe stress analysis software PASS/START-PROF!

Try to work with the program yourself, download the fully functional 30-day trial version from passuite.com/trial!

PASS/START-PROF Installation Steps

If you have any questions, please ask developers directly using support@passuite.com, using web form or using social media Facebook, Linkedin.

  • Step 1: Open passuite.com/trial in your browser and download PASS/START-PROF Trial by clicking the following link:

Step 2: Install PASS/START-PROF Trial. Please read the Quick Start Guide to see how to do it. Run PASS/START-PROF. Start > Programs > START-PROF > Start-Prof

Step 3: Enter email, click the “…” button. Receive the email from PASS LMS and click the link inside the e-mail. Copy the confirmation code and paste it in in registration card program and press OK.

PASS/START-PROF will start working immediately. You will see the following screen

Troubleshooting Steps

If you receive the message about problems with internet connection and your company has the proxy, try to enable it in the registration form:

Also at the corporate firewall you need to enable the incoming and outgoing traffic over HTTP:80 and UDP:6001 ports to the machine where START-PROF is installed. To test the port 80 traffic please open the passuite.com web site on the machine.

Email with code was not received

If you didn’t receive the e-mail, please wait 10-15 minutes and check your spam folder. Some corporate email servers may block the emails from LMS@passuite.com due to it’s settings. Passuite.com email server should be added into the white list. In this case please try to use another e-mail, for example gmail.com or write to support@passuite.com or using the web form, we will help to resolve this.

In case of any problem you can ask developers directly using support@passuite.com, using web form or using social media Facebook, Linkedin.

Some of the typical problems with solution are described below.

Error message #60 “SERVER_PROBABLY_NOT_UP” or #67 “NO_SERVER_RUNNING”

It means that PASS/START-PROF can’t reach the license management server (LMS) through UDP/6001.

Possible reasons and solutions of Error #60:

  • Windows Firewall or corporate firewall or internet provider firewall blocks incoming or outgoing traffic via UDP/6001. Change firewall settings to allow all incoming and outgoing traffic through UDP/6001
  • If failed, you may try to use mobile internet from your cell phone using the wi-fi hot spot to overcome the restrictions and firewall that block the UDP/6001 traffic on your laptop
  • If corporate firewall restrictions can’t be changed, then try to run PASS/START-PROF Trial from home
  • Server is down. Please try again after 10min – 1 hour. If you still see the same error then contact our support

For any other problems that are not described above you may ask developers directly using support@passuite.com, using web form or using social media Facebook, Linkedin.

Error message “Internet access is absent”

It means that PASS/START-PROF can’t reach the license management server (LMS) through HTTP/80. To test the port 80 traffic please open the passuite.com web site on the machine.

  • Windows Firewall or corporate firewall or internet provider firewall blocks incoming or outgoing traffic via HTTP/80. Change firewall settings to allow all incoming and outgoing traffic through HTTP/80
  • Internet access is going through corporate proxy server. Need to specify the settings of the proxy server in registration form
  • If failed, you may try to use mobile internet from your cell phone using the wi-fi hot spot to overcome the restrictions and firewall that block the UDP/6001 traffic on your laptop
  • If corporate firewall restrictions can’t be changed, then try to run PASS/START-PROF Trial from home

What to do after installation?

Enroll the complete 25 hours Pipe Stress Analysis Training Course!

Once the installation is finished you can open the example models and review analysis results as shown below

Follow the video for creating any simple piping model:

Watch the workshop recording and try to repeat:

  • 00:00 Intro
  • 14:13 How to model the vessel nozzle
  • 16:00 How to model the nozzle flexibility using WRC 297
  • 21:02 What is the local and global flexibility
  • 23:00 How to reduce the loads on equipment
  • 35:17 How to model the nozzle flexibility using FEA
  • 38:47 How to check the nozzle stress using WRC 531/297, FEA
  • 41:35 How to model the pump, compressor, turbine
  • 50:30 How to check the loads on the pump, compressor, turbine
  • 51:12 How to consider the more accurate SIF and k-factors according to ASME B31J
  • 53:00 Smart tee model according to ASME B31J
  • 01:00:50 How to model the tank nozzle: settlement, bulging effect, thermal expansion, flexibility
  • 01:03:53 How to check the loads on the tank nozzle using API 650, STO and FEA
  • 01:07:45 How to show working only one pump in the same time
    01:09:52 How to take into account the various operating modes with different P, T, etc.
  • 01:16:23 Why pipe never returns to installation state and friction forces are not zero
  • 01:17:50 Why stress range should be calculated from hot to cold state instead of installation state
  • 01:20:45 How to avoid zero intermediate anchor loads
  • 01:25:28 How to model the buried piping
  • 01:41:25 How to analyze piping for the water hammer loads
  • 01:47:30 How to add the seismic loads

What is Produced Water? | Produced Water Treatment Processes

Written by Anup Kumar Dey in Piping Interface,Process

Produced Water Source

Hydro Carbon reservoirs always have water along with crude/gas. The water(brine) brought to the surface from the hydrocarbon-bearing reservoirs during the extraction of oil and gas can include formation water, injection water, and any chemicals added downhole or during oil/water separation processes is termed “Produced Water”. The produced water is very high in salt content and also contains oil in dispersed form and therefore needs to be treated before it is disposed of. This article will mostly concentrate on the Conventional Methods for Oil Removal from Produced Water.

Contaminants in Produced Water

Contaminants in Produced water
Fig. 1: Contaminants in Produced water

Produced Water Disposal

The major disposal/re-use of Produced Water are

  • Re-Injection for Pressure Maintenance
  • Injection in Disposal wells
  • Overboard Discharge

Every disposal mechanism has quality criteria, and the same shall be met before disposal. The typical disposal criteria are as given below:

CriteriaWater InjectionInjection in Disposal WellsOverboard Discharge
OiW (ppm)~10 ppm10-40 ppm40 ppm-100 ppm#
Table: Typical Disposal Values for Produced Water

Produced Water Cycle

A typically produced water cycle is shown in Fig. 2

Produced Water Cycle
Fig. 2: Produced Water Cycle

Classification of Produced Water Treatment Process

Refer to Fig. 3 which explains the classification of Produced Water.

Treatment Processes
Fig. 3: Treatment Processes

Produced water Treatment Processes

Produced water Treatment using SKIM TANK (Fig. 4):

  • Principle: Gravity Separation
  • Description and Operation:
    • Designed to provide long residence times.
    • Their design can include a simple tank with water inlet/outlet baffles and an oil skimmer.
    • In some designs, the well fluid first flows into a degassing chamber. Liquids flow down into the tank, and exit under a cone or spreader.
    • In some designs, the oil and water compartments are separated within the tank and the skimming is ensured by Interface control.
Typical sketch of Skim Tank and coalescers
Fig. 4: Typical sketch of Skim Tank and coalescers

Advantages and Disadvantages of Produced Water Treatment using Skim tank:

  • Provide large surge capacity ensuring stable flow to downstream
  • Large residence time also ensures solids separation
  • Require Large Footprint area
  • Produced water channeling (short-circuiting) recirculation or stagnant areas of fluid can occur due to poor internal design or build-up of solids.

Produced water Treatment in COALESCER (Fig. 4):

  • Principle: Coalescing and Gravity Separation
  • Description and Operation:
    • Coalescer includes installation of Coalescer Plates or Packs in the conventional separator
    • The pack provides more surface for suspended oil droplets to coalesce into larger globules.
    • In the separation process, the oil droplet size is very significant, therefore it is also installed upstream of separation devices to increase the Oil particle Sizes and thereby the efficiency of the downstream equipment.

Advantages and Disadvantages of Produced Water treatment in Coalescer:

  • No moving parts and simple control
  • Little maintenance or attention required
  • Can handle high levels of oil in the produced water
  • Capable of handling relatively large oil content fluctuations
  • Plates can block with solids or fouling hydrocarbons.  

Produced Water Treatment in API SEPARATOR (Fig. 5):

  • Principle: Gravity Separation
  • Description and Operation:
    • Fluid Enters at one end and separates along the length as per density.
    • Most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to the top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom.
    • Typically, the oil layer is skimmed off and re-processed or disposed of.
    • The bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump.
    • The water separated is sent for further processing.

Advantages and Disadvantages of Produced water Treatment in API Separator:

  • Simple in design
  • Large residence time also ensures solids separation
  • Is a very old design and has been replaced by a CPI separator
  • Solids removal from the bottom is difficult and separated solids affect the separator’s efficiency
  • Large footprint area
  • Atmospheric Design-Cannot be used for PWRI
  • Need a degasser to be installed upstream

Using CORRUGATED PLATE INTERCEPTOR (CPI-Fig. 5) for Produced Water Treatment:

  • Principle: Gravity Separation
  • Description and Operation:
    • Fluid Enters at one end and separates along the length as per density just like an API Separator
    • CPI includes corrugated plates arranged in a plate pack which is installed at an angle of 45°
    • The plate provides more surface for suspended oil droplets to coalesce into larger globules.
    • Separated solids would slide down and separated oil drops move upwards due to their lesser density than water.
Typical Sketch of API Separator and Corrugated Plate Interceptor
Fig. 5: Typical Sketch of API Separator and Corrugated Plate Interceptor

Advantages and Disadvantages of Corrugated Plate Interceptor in Produce Water Treatment:

  • Corrugated plates enhance the degree of oil-water separation and therefore it requires significantly less space than a conventional API separator
  • Atmospheric Design-Cannot be used for PWRI
  • Need a degasser to be installed upstream

Produced Water Treatment in HYDRO-CYCLONES (Fig. 6):

  • Principle: Centrifugal Force
  • Description and Operation:
    • Fluids enter the hydro-cyclone through the tangential inlet at the top which causes the fluids to spin and attain High Centrifugal forces
    • The centrifugal forces in a hydro-cyclone are of the order of several hundred times normal gravity force. This promotes rapid separation.
    • The clean water exits the Hydro-cyclone through the open end of the Tube at bottom
    • A vortex finder in the outlet port reverses the direction of the hydrocarbon core and it is discharged under differential pressure control, from the reject port at the center of the Inlet End
    • In order to maintain hydro-cyclone separation efficiency, the Pressure Differential Ratio (PDR) must be maintained within certain limits. The PDR is defined as follows: PDR=DPir/Dpio Where, DPir = Pressure at the inlet – Pressure at reject outlet; DPio = Pressure at the inlet – Pressure at water outlet; Oil removal efficiency will decline <1.5=1.7-2.0=<Volume of liquid routed to the reject increases

Advantages and Disadvantages of Produced water Treatment in Hydro-Cyclones:

  • Compact in design
  • High Efficiency @ particle size less than 10microns
  • Modular Design gives flexibility for capacity enhancement
  • The energy Requirement to pressurize Inlet is high
  • High cost due to the Metallurgy-Erosion Issues

Using Induced Gas Floatation(IGF-Fig. 6) for Produced water Treatment

  • Principle: Froth Floatation
  • Description and Operation:
    • IGF units operate by generating small gas bubbles and releasing these in the lower section of a vessel through which the water is flowing.
    • These bubbles attach themselves to the dispersed hydrocarbon droplets, reducing their density and floating them to the liquid surface.
    • The separated hydrocarbon is then removed and the treated water exits the vessel under level control.
    • There are two methods of Inducing bubbles viz. Mechanical and Hydraulic
    • The Mechanical method generated gas bubbles by means of motor-driven diffusers/impellers.
    • The unwanted side effect of the shear forces required to generate the micron-sized gas bubbles was a reduction in the size of the oil droplets which adverse effect on the overall efficiency of these units.
    • The Hydraulic method uses a stream of clean water from the outlet and mixes it with the gas from the vessel top in an eductor. The mixed stream is then injected into the floatation cells through nozzles.
    • Hydraulically inducing the bubbles results in lower shear forces in the flotation cell

Advantages and Disadvantages of Induced Gas Floatation for Produced Water Treatment:

  • Low Capex
  • High Inlet concentration can be acceptable
  • Relatively insensitive to changes in oil droplet size
  • Require steady flow for the effective operation
  • Normally requires a de-oiling chemical to be dosed upstream to optimize performance –High OPEX
Hydro-Cyclones and Induced Gas Floatation
Fig. 6: Hydro-Cyclones and Induced Gas Floatation

Produced Water treatment by Dissolved Gas Floatation(DGF-Fig. 7):

  • Principle: Froth Floatation
  • Description and Operation:
    • Bubbles are generated by saturating a liquid stream with gas, typically at a pressure of 44 to 87 psig (3 to 6 barg).
    • As the liquid enters the Flotation Chamber it is depressurized and the gas is released as fine bubbles.
    • The rising bubbles attach themselves to hydrocarbon and float them to the water surface from where they can be removed by a skimmer and weir arrangement.
    • The main advantage of DGF is that the method of producing bubbles is relatively gentle. The absence of the High Shear Forces helps in better separation.

Advantages and Disadvantages of Produced Water Treatment by DGF:

  • Proven technology
  • Moving parts and associated maintenance requirements
  • De-oiling chemicals are normally dosed upstream to optimize performance
  • Gas solubility decreases with increasing temperature which can make the technology less effective at higher operating temperatures

Compact Floatation Unit (Fig. 7):

  • Principle: Froth Floatation and Cyclonic Effect
  • Description and Operation:
    • The CFU combines the swirling tangential flow effect of a hydro-cyclone within a gas flotation unit.
    • The inlet fluid enters the vessel via one or more tangential inlets, establishing a mild rotation of the liquid in the vessel.
    • Nitrogen or fuel gas is introduced into the vessel via a bottom distribution nozzle or “sparger” (there are some differences between vendors regarding how the gas is introduced into the vessel).
    • The oil rejects from the vessel are normally removed via a central weir pipe at the top of the vessel.

Advantages and Disadvantages:

  • The residence times in CFU’s are significantly lower than traditional IGF systems, with residence times of 1 minute being typical, compared to 4 minutes for IGF systems.
  • Significantly smaller and lighter than conventional IGF
  • Excellent turndown
  • Normally requires a deoiling chemical to be dosed upstream to optimize performance
  • Sensitive to vessel motion

Produced Water Treatment using Crushed Nut Filter (Fig. 7):

Principle: Hydrophillic Nature of Crushed Nut Shells

Description and Operation:

  • Nutshell filtration involves the removal of suspended hydrocarbon liquids and solid matter from wastewater by passing the water through a bed of crushed nutshells(Typical-Walnut)
  • The crushed nut shells have an affinity to hold the oil particles and the suspended particles.
  • As the impurities are absorbed the media gets choked resulting in a higher pressure drop.
  • This is an indication that the filter needs to be taken for backwash
  • Once backwashed, it is again ready for filtration. To keep the process uninterrupted the filters are with N+1 configuration to take care of the capacity reduction during backwash.

Advantages and Disadvantages of Produced Water Treatment using Crushed Nut Filter:

  • High-quality water effluent
  • Very efficient for IW
  • Removes TSS in addition to OiW
  • The backwash mechanism is high on Energy and Maintenance
  • Erosional issues due to the abrasive nature of media
  • Large and heavy equipment
DGF, Compact Floatation Unit and Crushed Nut Filter
Fig. 7: DGF, Compact Floatation Unit and Crushed Nut Filter

Use of Dual Media Filter (Fig. 8) for Produced Water Treatment:

Description and Operation:

  • In a multimedia filter, two or more media with different grain sizes and densities are used.
  • Within each layer, there will be size segregation with larger particles at the top and smaller at the bottom.
  • The advantage is that this helps to minimize blockage and also maximize bed depth utilization.
  • As the impurities are absorbed the media gets choked resulting in a higher pressure drop.
  • This is an indication that the filter needs to be taken for backwash
  • Once backwashed, it is again ready for filtration. To keep the process uninterrupted the filters are with N+1 configuration to take care of the capacity reduction during backwash.

Produced Water Treatment Future Development

  • MEMBRANE FILTERS
  • CENTRIFUGE
  • CARTRIDGE FILTERS
  • VENDOR SPECIFIC DESIGN
Dual Media Filter
Fig. 8: Dual Media Filter

Piping Stress Man-hour estimation process for a specific Project

Written by Anup Kumar Dey in Piping Stress Analysis,Piping Stress Basics,Project Management

Man-hour is the unit for estimating the time required for completing an activity of a project. Man-hours estimation is a crucial activity for winning projects. Labor cost constitutes a considerable amount of the project cost and the same is calculated by estimating man-hours.

Importance of Man-Hour Estimation

Estimating the project man-hour is a very critical activity and should be done very accurately. Any mistake in this action may result in a huge loss to any organization. If more man-hours are quoted then the project may not be awarded to the organization and if less man-hour is quoted then the design company may incur a cost in the long run. So one should make a competitive man-hour estimate considering all activities.

Piping stress analysis is a very important activity in piping design and separate man-hours must be claimed from the client for this. Normally, on average, piping stress analysis-related activities require roughly 8-12% of the total piping man-hour. The man-hour estimation is done either during the project bidding activity or during the initial phase of any project. So hardly any firm data is available while man-hour estimation is done. So sufficient cushion needs to be added for rework activity. The following write-up will provide a guideline for the piping stress man-hour estimation process for any project. Broadly, there are three practices for man-hours calculation that are followed in established EPC organizations. They are:

Piping Man-hours estimation
Piping Man-hours estimation

1. Estimating Piping Man-hour based on Stress Packages

Normal practice is to estimate the number of stress packages from available P&ID, then multiply the same by some constant value (decided based on experience). Few organizations divide stress packages into three groups,

  • highly critical stress systems
  • moderately critical stress systems and
  • lower critical stress systems

Lets for example if for a project 250 stress packages are estimated out of which 20 are highly critical, 100 are moderately critical, and rest 130 are lower critical then manhour estimates will be =1.2 X (20×50 + 100×40 + 130×30) =10680 considering 50 man-hours for a highly critical package, 40 man-hours for a moderately critical stress package and 30 man-hours for lower critical stress package. The total value is increased by 20% as the data is not firm.

2. Calculating Piping Man-hour based on Stress Critical Lines

In this method, the total number of stress critical lines is estimated from the existing P&ID. Then that value is multiplied by a factor (Normally 12 to 18 depending on the experience of the existing organization). For example, if the project mentioned above contains 4 stress critical lines (you have to calculate critical lines from P&ID) per stress system on average, then the total number of stress critical lines is 250×4=1000. So total piping man-hours estimated would be approximately 1000 x 12=12000.

3. Estimating Man-hours based on Critical equipment

This method is somewhat similar to the method mentioned in point no 1. In this method, the total number of equipment is calculated from the P&ID. All equipment is then categorized into three groups

  • Highly critical equipment like Turbines, Compressors, High-temperature Reactors, Furnaces, etc.
  • Moderately critical equipment like Air Fin Cooler, Pumps, High-temperature Columns, Heat exchangers, etc. and
  • Lower Critical equipment like Low-temperature Columns, Heat exchangers, etc.

Then the number of equipment is multiplied by the same predecided (based on experience) factors to get the total man-hours.

Documents required for Man-Hour calculation

In any of the above situations the following input documents are required at a minimum:

  • P&ID (If not available then man-hour estimation can be done based on preliminary P&ID or FEED P&ID)
  • Line List, if available
  • Equipment list, if available

However, in my opinion, one should look into the exact scope of work (activity-wise) by the stress department. Then one should consider reasonable man-hour (which normally takes based on experience) for each activity. And finally, add all the man-hours of each activity together to get the final man-hour. This way one could get the man-hour close to the actual.

4. Man-hour Estimation Based on Activities Performed

The first step will be to list out all the activities in the stress scope. For example from the initial start of any project broadly following stress activities have to be considered:

1. Study of Project Specifications and bid documents
2. Work Instruction and Specification preparation from bid documents if required
3. Pipe Rack Loading (Three or Four Stages)
4. Stress System definition from P&ID
5. Master Critical Line list preparation (Will be updated 3 to 4 times during project tenure)
6. Preliminary stress analysis before receipt of vendor equipment data (without proper documentation)
7. Final stress analysis after receipt of vendor equipment data (With proper documentation)
8. Spring datasheet and Spring Index preparation
9. Special support design
10. Datasheet preparation for miscellaneous items like an expansion joint, sway brace, strut, etc
11. Trunnion calculation
12. Review of Stress systems
13. Vendor communication
14. Interdepartmental communication with the Civil, Mechanical, and Process departments
15. Technical Bid Evaluation of Spring Hangers and Special items
16. Job close-out report preparation
17. Support Checking of critical lines
18. Support checking of Non-critical line
19. Isometric review and issue
20. Other miscellaneous activities which are not listed above

After listing all such activities one needs to decide on actual time factors for each activity based on experience and then finally combine all those to get the final man-hour.

Hot Sustained Stress (Lift-Off) Checking in Caesar II

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

What is Hot Sustained Stress?

In Layman’s terms, Sustained means always present. So sustained stresses are the stresses which are present in the system throughout its operating cycle. The weight of the piping system and Pressure inside the pipe are examples of sustained loads that generate sustained stresses in the system. So what is a hot sustained case?

While analyzing a piping system, many times you will come across some supports which will take a load in sustained cases but are not taking a load in operating and design temperature cases (Refer to Attached Fig. 1 and Fig. 2 for one such typical example). The support is lifting at that point in temperature case i.e. supports are not contributing to load and stress distribution while in operating condition. Still in that situation, the weight of the pipe and pressure inside the system will induce sustained stresses. So in my opinion, hot sustained stress is the sustained stress in pipe operating situations. And we must ensure that the system stress will not fail because those supports not sharing any load. That is why many organizations make it mandatory to check sustained stresses.

hot sustained 1
Fig.1: Caesar II Restraint summary showing lifting supports

Methods of Hot Sustained stress checking

I have come across two different methods of hot sustained stress checks in the various organization based on old philosophy:

1. Conventional Method of Hot-Sustained Check

  • In the first method, the analyst has to run the static analysis as per the conventional method.
  • Now go to the restraint summary and note down the support nodes which are lifting or not taking any vertical load (Sometimes small positive value may be there due to guide and line stop frictions, in that case, check the vertical displacement if it shows positive value consider the same as lifting).
  • Make a separate Caesar file with the name FILE NAME_HOT SUSTAINED.
  • Open the input screen and delete all lifting supports from the nodes you noted down. Delete only +Y support, Guide and line stops will be there.
  • Run the analysis and check sustained stress.
  • If sustained stress is within allowable limit accept the file as it is else change the support location or routing to make the system safe.
hot sustained 2
Fig.2: Caesar II Plot showing lifting support

2. Second method of Hot Sustained Checking

  • In this method, the analyst will check the hot sustained stress in the same main file (No need to create a separate file). Some additional load cases are required. Let’s assume we will check hot sustained stress in design temperature, T1 condition (means we will check which supports are lifting in design temp case). So the below-mentioned cases are required for hot sustained stress checking

L1:                                W+T1+P1                                     OPE
L2:                                        T1                                OPE/EXP
L3:                                  L1-L2                                          SUS

  • Check the stresses for load case L3, if the same is within the allowable limit then accept the file else make the system safe.

3. Hot Sustained Check Philosophy from Caesar II 2018 version onwards:

From Caesar II version 2016 onwards, based on code requirements they have created a load case type known as ALT-SUS. So, in the single file, by adding ALT-SUS load cases, Caesar II automatically calculates hot-sustained stresses for each operating condition. Refer to Fig. 3 below to understand the load cases which has been introduced from ASME B 31.3-2014 onwards as Alternate sustained stress checking.

Alternate Sustained Load Cases
Fig. 3: Alternate Sustained Stress Checking

So, the above stresses are sufficient for hot-sustained stress checking. Load case L3 is checking sustained stress for temperature T1; which means in load case L3 all supports which are lifting or not taking load will be removed by the software automatically and sustained stress will be calculated after that. A similar situation will arise for load cases L5 and L7 as shown above and they are the hot sustained cases with respect to temperature cases T2 and T3 respectively.

Notes:

  1. Now you may be thinking about whether to mark deleted supports in isometric or not. You must mark those supports. As we have not deleted the supports in actual practice. Supports will be there at site, We simply ensured that without those supports also the system will be safe. However, if you want to delete those supports that can be done if all other stress criteria can be met.
  2. Whether we need to check expansion stresses in a hot sustained file too? In my opinion, if we are using liberal stress for expansion stress range checking then it is better to check expansion stress (along with sustained stress) in the hot sustained file. Otherwise, it is not required as the system won’t fail in an expansion cases even after removing those supports.