This document covers the requirements for the determination of design pressure and design temperature of the equipment like pressure vessels and piping that are used in petroleum refineries, petrochemical plants, and other similar plants.

The following summary of input data needs to be prepared prior to the determination of design conditions.

Operational data

• -Service for identification
• -Operating temperature & pressure under normal operation
• -Density of the fluid
• –Pressure drop of internals
• -Static head for vessel & lines
• -Differential pressure of pump at shut-off or design condition
• -Operating conditions other than normal operation such as start-up, upsets, shut-down, etc.

Construction data

• -Line class or general pressure-temperature rating of flanges
• -Elevation at the inlet and outlet of the piping system
• -Location of valves that cause shut-off conditions

Client Design Philosophy (As part of ITB)

Output data:

• -Design Pressure & Temperature for the vessels
• -Design Pressure & Temperature for the pump discharge lines
• -Short-term design condition, if required
• –Minimum design temperature

Design Pressure of Pressure Vessel:

The design pressure for the pressure vessel is determined in the following way,

• -Unless otherwise specified, this indicates gauge pressure. If design pressure<Atmospheric pressure, then absolute pressure may be used in place of gauge pressure.
• -Design pressure shall be based on the expected maximum pressure drop at the top of the vessel under normal operation.
• -When maximum pressure under normal operation can’t be estimated, the design pressure should be determined as per the table (Table 1) below.
• -When the vessel is venting to the atmosphere, the minimum design pressure shall be ‘’Full of Water’’ or “Full of Liquid” (whichever is greater).
• -When the vessel is connected to a flare, the minimum design pressure shall be 3.5 kg/cm²-g.

Design Pressure Estimation

• -The design pressure should be indicated in the specification as “Design Pressure at Top”.
• -The maximum pressure at the bottom of the vessel is calculated by the mechanical design department to determine the wall thickness of the vessel.
• -It is thus necessary to represent the height and specific gravity of the liquid and also the pressure drop through the internals.

Example 1:

                                                                     Operating Pressure                                            Design Pressure

Pressure at the top (kg/cm²g)                                     2                                                                         3.8

Pressure drop (kg/cm²)                                       0.3                                                                      0.3

Bottom liquid head (kg/cm²g)                            0.2                                                                     0.2

Pressure at the bottom (kg/cm²g)                    2.5                                                                     4.3

As a practice, while a vacuum condition is likely to occur due to malfunction during steam purge or emergency shutdown of reboilers (all condensable vapor service), etc, the vessel should be designed to withstand full vacuum. If the vacuum design is an uneconomical solution, vacuum protection devices such as breather valves can be installed by employing vacuum design, keeping in mind the types, location, and response time of devices, flow rate, availability of inert gas is used as well as it should be ensured that the devices are functioning properly even under abnormal conditions.

When the vessel is located on the discharge side of the pumps and is not protected by relief devices, the design pressure shall be determined as the larger of the following criteria,

Design pressure= Differential pressure under normal flow rate + design pressure of suction vessel+ head between the tangential line of the suction vessel and the centerline of the pump impeller.

Design pressure= Pump shut off head +normal operating pressure of suction vessel+ head between the tangential line of the suction vessel and the centerline of the pump impeller. The pump shut-off head can be calculated as Maximum suction pressure + 1.25 x Normal differential pressure.

For the centrifugal compressor, the design pressure of the compressor discharge shall be at least equal to the specified relief valve setting, if the relief valve setting is not specified, the design pressure shall be at least 1.25 times the maximum specified discharge pressure (refer API RP 617). A safety valve must be used on the discharge of each stage of a reciprocating compressor to avoid possible damage to the machine from excessive pressure due to overloading.

Design Pressure of Heat Exchanger

The cause of the over-pressurization of the heat exchanger should be known before determining the design condition of the heat exchanger. The causes are as follows,

• -Blocked Inlet: Thermal expansion of cold side fluid
• -Blocked Inlet: Vaporization of cold side fluid
• -Blocked Outlet: Hot side or cold side
• -Internal Tube Failure: Low-pressure side

The below points are a measure for the protection of the heat exchanger due to over-pressurization,

• -Install a Pressure safety valve to limit over-pressure
• -Raise the design pressure to eliminate or mitigate one or more overpressure cases. For example, design the exchanger for pump shut-in pressure to eliminate the blocked outlet case or design the low-pressure side for ten-thirteen (10/13) of the design pressure of the high-pressure side to mitigate the tube failure case.

Equipment Design Pressure

The below table (Table 2) can be used to determine the design pressure of the vessel/columns/Reactors.

The below table (Table 3) can be used to determine the design pressure of the casing & discharge piping of a compressor (ref API RP 617).

Note 1: As per API RP 617, for a reciprocating compressor with a rated discharge pressure above 5000 PSIG, the relief valve setting shall be as agreed to by the purchaser and the vendor.

The below table (Table 4) can be used to determine the design pressure of the heat exchanger.

The below table (Table 5) can be used to determine the design pressure of the fire heater

The below table (Table 6) can be used to determine the design pressure for the Relief and Flare system

Design Temperature:

Design temperature is determined based on the maximum normal operating temperature and the addition of a design margin. If temperature fluctuation is expected during normal operation, the maximum value of the fluctuating temperature must be considered. If the design temperature at the bottom of a pressure vessel is significantly different from that at the top, both temperatures should be specified. The design temperature shall be calculated as follows,

Minimum Metal Design Temperature (MDMT):

A minimum design metal temperature is the lowest temperature caused by depressurization. It should also be specified in the vessel data sheet. For most of the vessels, heat exchangers, pumps, and compressors, the ASME Boiler & Pressure Vessel Code, Section VIII, Division 1, or Division 2 is used. The division 2 code is normally used only for vessels of heavy wall construction, such as the reactors in hydro-treating plants.

The MDT in Division 1 is called the “Minimum Design Metal Temperature” (MDMT), while the term “Minimum Permissible Temperature” (MPT) is used for Division 2.

Steam out Design Temperature

It is recommended that a minimum design temperature of 120ᵒC accommodate the steam out temperature.

Difference between Design Pressure and Maximum allowable Working Pressure

The difference between the design pressure and Maximum allowable working pressure can be described as below,

What is a Process Flow Diagram?

A Process Flow Diagram (PFD) is a simplified diagram that shows the process flow of a manufacturing process in proper sequence. This diagram should consist of every essential detail like main equipment, Heat, Material, & Energy Balance, tag number, chemical composition, etc.

Also popular as Process Flow Chart, a Process Flow Diagram (PFD) describes the relationships between major components at any chemical, process, or power plant. Process Flow Diagrams or PFDs are developed using a series of symbols and notations to convey information for a process. The concept of PFD or process flow diagram was first introduced to ASME by Frank Gilbreth, Sr. in the year 1921.

Purpose of PFD or Process Flow Diagram

The purpose of the process flow diagram is to define the design of the process.  A PFD is a fundamental representation of a process that schematically shows the conversion of the raw materials to the final products. Also, PFD is a basic document of a project as it is required by the project design team during the developmental stages of a project. At these stages, feasibility studies, and scope definition activities are undertaken before commencing detailed design. PFD is very closely associated with H & MB.  For a project to proceed, they are used to decide if there are enough raw materials & utilities. The PFDs are documents that are used by plant-wide design groups and site management in a manufacturing organization. On completion of a PFD, detail engineering starts. Some engineering contractors or owners use a process flow diagram as a base for designing the instrumentation & control scheme. Other benefits that a process flow diagram serves are

• PFDs show the plant design basis indicating feedstock, product, and main stream flow rates and operating conditions.
• A PFD documents a process for training, quality control, and simpler understanding.
• It standardizes a process for optimal efficiency and repeatability.
• The scope of any process can easily be found in its PFD diagram.
• It provides an overall idea of the complete process without detailing much on the minute details.
• It helps to study the process in a simple way for better communication and brainstorming.
• PFDs serve as the input document for the creation of P&IDs.
• PFD diagram also provides information regarding the utility services that will be continuously used in a process.

Development of Process Flow Diagram

A PFD is most likely developed in multiple steps. The plant owner may develop a preliminary PFD, as a first step that sets down on paper a proposed process or a process change that is under consideration. But nowadays a PFD is generated by computerized simulator software using a library of indicating symbols like follows,

• Advanced simulation library,
• Aspen Hysys, Aspen Plus,
• CHEMCAD,
• CHEMPRO,
• DynoChem,
• DYNSIM,
• DWSIM,
• Flow Tran,
• PIPESIM,
• OLGA,
• Petro-SIM,
• ProMax,
• PRO/II, etc.

For a standard process, there are several licensors like UOP, Axens, Lummus, etc. & they are responsible for developing the PFD.  Process flow diagrams of open art process units like CDU (Crude distillation unit), and VDU (vacuum distillation unit) can be found in the literature and encyclopedia of chemical technology. Some of them may be obsolete. After that, the PFD is reviewed by the engineering contractor’s process engineer & planning team before the release of the detailed design. So there is enough information in the PFD like material balance information, main equipment, etc. PFD undergoes several revisions based on the review, cost optimization, HAZOP study, opinion of the expert team, etc.

What are the basic information that a PFD contains?

The depth of information furnished in a process flow diagram may vary from organization to organization. However, in general, the following information is added in a PFD.

Design basis:

For batch process, indicate batch capacity & batch cycle time. For continuous process indicate design production rate.

Material balance:

Composition & quantity of material in process for each unit operation & each important pipeline. For batch process indicate equipment capacity & pipeline flow per batch basis. Continuous processes show flow rates in weight or volume units per hour.

Energy balance:

Indicate heat balance or heat transfer data for each unit operation.

Physical data:

Indicate operating pressure and temperature, specific gravity, molecular weight, viscosity, specific heat, and other important data.

Equipment information:

Represent all the unit operations involved in the process. Spare equipment, duplicate parallel lines, and bypass lines don’t need to show unless necessary. Arrange all the unit operations properly so that maximum simplicity can be achieved. Indicate the correct relative position so as to gravity flow, seal loop, and other requirements can be represented correctly.  Represent the proper name, tag number, and capacity of each equipment item involved in the unit operations.  Important features of the equipment like agitators, trays, and jacket coils are to be indicated in PFD.

Piping information:

Major process lines associated with the process are to be shown with proper directions.  Other important piping items like sampling point, inline filter static mixture, etc as well as control valves to be shown in a process flow diagram. Show the major service or utility line that is essentially required for the process.

Instrument Information:

Show all the critical instruments which are involved in the control loop like the mass flow meter, temperature transmitter, etc. Along with the proper control scheme the instruments participated in the control scheme to be shown in the process flow diagram.

Size and Scale:

Use the standard scale ratio set by project standards for drawing the PFD. Generally, arrange the flow for trimming to 11” or 22” height. If an absolute scale is not used the process flow diagram should show the relative size and elevation of the components.

Drawing Instructions:

Use the standard symbol of the equipment, sketch, etc, set by the project standard. It should be represented in the legend sheet. Use the applicable data in a tabulated format and show it at the bottom of the process flow diagram for each unit operation and process line. Define the steam numbers of each process line shown in the process flow diagram whose operating condition should be represented in the process flow diagram. Indicate the proper arrow for showing the correct flow of the process so that the process flow diagram can be easily understood.  Mention the process lines which carry the two-phase flow.

Flow summary:

Process flow data & conditions are provided on the process flow diagram (ref Table 5). As the process flow diagram is related closely to the material balance, mass flow units are normally used. Additionally, pressure & temperature conditions are provided as well. Flow summary is shown on the process flow diagram which contains all the necessary information.  The below-tabulated format (Table-1) that contains the following information about the process should be represented in the process flow diagram.

As already informed that PFD is not detailed drawings of the process. So It does not provide the following information, in general:

• Process control instruments
• Instrumentation of trip system
• Pipeline numbers with classes
• Shutoff, Isolation, and Minor bypass values
• Vents and drains
• Relief and safety valves
• Equipment dimensional information and requirement of spare equipment
• Code class information, etc

Symbols for Process Flow Diagram

The process flow diagram shall use symbols & letter designation to represent the equipment on the process flow diagram. It is not at all necessary to add more details to the equipment shown on a process flow diagram. For example, a heat exchanger can be represented as a simple line representation of the main process flow and heat transfer medium flow, without implying a particular type of exchange. For a process flow diagram, the only information required is that a piece of equipment transfers heat at that point of the process rather than showing specifically the mechanism for heat transfers.

Stream Identification

Conventions used for identifying process equipment

The below tables (Table 2) contain the symbolic letters which are generally used to identify the process equipment in a process flow diagram.

Process flow diagram Symbols for Equipment | PFD Equipment Symbols

The process flow diagram symbols for equipment are shown in Fig. 2 below.

Equipment Description for PFDs and PIDs

Example of Process Flow diagram

As per Fig-3, it is clearly showing that there is a flow in the process line, stream number (1) of 10000 lb/hr of wet gas with a temperature between 90 F and 180 F and a pressure of 20 Psi. The variation in temperature is caused by a process upset at the upstream of PFD. Note that, only a stream number, (1),(2),(3) identifies the pipeline. Not included is the line size, the material of construction, or the pressure rating (ANSI 150, ANSI 300, etc) for any of the piping shown on the PFD. Also, note that there is no symbol or data shown for the pump driver. Only the equipment number, G-005 identifies the pump. So it is very clear that the PFD represents the relationship between the main equipment of a process plant and at the same time, it does not show minute details like piping details and designations, etc.

Flow summary table:

Codes and Standards for Process Flow Diagrams

The following codes and standards can be used for developing process flow diagrams for the process industry.

• ISO 15519
• ISO 10628
• SAA AS 1109
• ISA 5.7

Are PFD and P&ID different?

Yes, both PFD and P&ID are two different process documents. P&ID provides more detailed information about the process steps. The main differences between a PFD and P&ID are provided here.

Online Courses on Process Flow Diagram

If you are planning to learn and read process flow diagrams and P&ID like a professional then check out the following course: How to Read P&ID, PFD & BFD used in Process Plant like Pro

Additionally, you can decide on the following course which is also very useful: Chemical/Process Engineering Drawings and Diagrams

What is an Ejector?

An Ejector is a piece of equipment often used to eject gases and vapors or non-condensable from a system to generate a vacuum. Ejectors are used in several industries in numerous ways including chemicals, pharmaceuticals, FMCG, petrochemicals & refineries, etc.

Working Principle of an Ejector

An ejector follows Bernoulli’s Principle i.e. when the kinetic energy of a fluid increases its pressure energy decreases to maintain total energy constant & vice versa. Equation 1 clearly indicates that if velocity increases pressure energy decreases.

P+(1/2)ρV²+ρgh=Constant……….(1)

Where,

• P= Pressure energy
• (ρV²)/2  = Kinetic energy per unit volume
• ρgh       = Potential energy per unit volume
• ρ            = Density
• V            = Velocity

The ejector has a converging section in which velocity increases to convert pressure energy into kinetic energy. This transformation results in a low-pressure region that provides the motive force to draw the process fluid. Then both the fluid is mixed & flows through the diverging section consisting of a diverging nozzle. As it propagates through the diverging section the kinetic energy converts into pressure energy by decreasing its velocity and increasing the pressure. Finally, by re-compressing the mixed fluid it meets the destination pressure.

How vacuum is created in an Ejector?

In the ejector, the velocity of the motive fluid becomes very high as it expands across the converging and diverging nozzles from motive pressure to the operating pressure of process fluid. The expansion of the motive fluid through the motive nozzle causes supersonic velocities at the exit of the nozzle. Velocity coming out from a motive nozzle is 3 to 4 times the Mach number. In the actual scenario, the motive fluid expands to a pressure lower than the suction process fluid pressure. This causes the driving force to draw the suction fluid into the ejector. High-velocity motive steam entrains and mixes with the suction fluid.

Main Parts of an Ejector

There are five main parts of an ejector which are as follows,

STEAM CHEST: This is the part of the ejector where the high-pressure motive fluid is entered into the ejector.

SUCTION CHAMBER: It is a chamber that has proper connections for the process inlet, diffuser section, and motive nozzle.

INLET DIFFUSER:  It is a properly shaped introductory section and converging diffuser zone that handles the high velocity of the fluids. In this section, entrainment and mixing of the motive and process fluids occur, and the energy of supersonic velocity is converted to pressure energy.

THROAT SECTION: This section is the transition section where the converging section ends and the diverging section begins. Basically, it is located at the junction of the converging supersonic inlet diffuser and the diverging subsonic outlet diffuser.

OUTLET DIFFUSER: It is a properly shaped diffuser section for accomplishing the conversion of velocity head to pressure head. After passing the fluid through the throat of the diffuser, the velocity becomes essentially subsonic. The outlet of the diffuser section further decreases the fluid velocity to a significant level so as to convert the kinetic energy to pressure energy.

Types of Ejectors

Ejectors are mainly categorized into two types which are as follows,

• Single-Stage ejector
• Multi-Stage ejector

Single-Stage Ejector

Single-stage ejectors are the simplest and most frequently used in industries. They are generally recommended to use for pressures from atmospheric to 80 torrs or for compression ratio <10. A single-stage ejector discharges at or near atmospheric pressure.

Multi-Stage Ejector

A multi-stage ejector is normally used when a generation of high vacuum is required that is normally from the atmosphere to in the range of 30 torrs to 0.05 torr. For the generation of such low pressure, up to six stages of the ejector can be used.

What is Motive Fluid?

Motive fluid is the fluid that motivates the process fluid to draw into the ejector. Normally, high-pressure steam is used as a motive fluid, but compressed air or gas can also be used as the motive fluid. The choice depends on the availability of the utility, operational feasibility, etc. A minimum pressure of the motive fluid is required to maintain a stable operation & thereby to design a stable ejector system. If the pressure of the motive fluid falls below the design pressure, then the nozzle will pass less steam than required. If it happens, the ejector is not provided with sufficient energy to compress the process fluid to the design discharge pressure. A similar problem occurs when the supply temperature of motive fluid rises above its design value, which results in increased specific volume, and consequently, less steam passes through the motive nozzle.

What is a Gas Ejector?

A gas ejector is an ejector which utilizes high-pressure gas as a motive fluid. The motive gas can be compressed natural gas, nitrogen, air, etc. Normally gas ejector has three connecting points high-pressure gas, low-pressure gas & discharge. This type of ejector is used to draw flare gas & routed it to flare.

What is a Steam Ejector?

Steam ejector is the ejector which utilizes high-pressure steam as the motive fluid. It has a converging and diverging nozzle across which pressurized motive fluid is passed. In the diffuser section, the velocity of the mixed fluid is recovered to pressure energy greater than suction pressure but it is lower than the inlet pressure of the motive steam. This pressure should be greater or equal to the backing pressure for smooth operation.  For low vacuum, multiple-stage ejectors are used. Table 1 shows the probable suction pressure vs total steam consumption in an ejector.

The steam jet ejector capacity is directly proportional to the weight of the motive fluid. Motive gas to process gas pumped is high, especially under low vacuum, and results in the huge requirement of steam in multi-stage systems. Operating parameter of motive steam such as inlet steam pressure, and discharge pressure has a significant impact on overall ejector performance.

Purpose of Inter-condenser

Inter-stage condensers & ejectors are staged in series with each other. The purpose of the inter-condenser is to condense hydrocarbon & steam as much as possible. The load of the downstream ejector can be reduced by condensing steam & hydrocarbon. So for proper maintaining of motive steam consumption condenser is highly recommended.

Factors affecting the performance of the steam ejector

The following factors have a significant impact on the performance of the ejector’s performance

• Motive steam
• Cooling water
• Dry & saturated air
• Gas & vapor densities

Motive steam:

A steam ejector is normally designed for a motive steam pressure of 15 to 600 PSIG. Motive steam pressure must be above a minimum pressure for stable operation. This minimum pressure is called motive steam pick-up pressure. So it is very much essential to maintain motive steam pressure; otherwise, the ejector cannot give the desired performance. On the other side, excess motive steam pressure can lead to the wastage of costly steam. Steam ejectors are operated normally with saturated dry steam or superheated steam.  5–15 °C superheating is recommended, but its effect should be considered during ejector design. The use of wet steam is not at all desirable, as it erodes the ejector nozzle and interferes with the ejector performance by clogging the nozzle with droplets of water. The design pressure of the motive steam should be selected as the lowest expected pressure at the steam nozzle of the ejector. The recommended design pressure of the steam is the expected minimum pressure at the motive nozzle: 10 psi.

Cooling Water:

In the inter-condenser, the temperature of the cooling water has a significant impact on the efficiency of the ejector. If the temperature of cooling water rises more than the design condition available LMTD of the condenser decreases.  In this situation, the condenser will not condense properly and vapor & non-condensable gases are carried out as saturated fluids.

On the other hand, if the flow rate of the cooling water decreases below the design condition, a huge temperature rise across the condenser occurs. Even if cooling water is at its designed inlet temperature, an increase in temperature rise reduces available LMTD across condensers. Thus the efficiency of condensation is greatly reduced, and additional load is passed on to the downstream stage of the ejector.

Dry and saturated air

If an ejector is used for maintaining a vacuum in the condenser, the air that is extracted by the ejector is saturated with water vapor. The entrained water vapor by the dry air that leaks into the condenser depends upon the temperature of the mixture and the vacuum at the ejector suction.

Gas and vapor densities

When an ejector handles chemical gases or vapors, it is necessary that the density of the gas be known. The compression of a given weight of heavy gas requires less operating steam than the same weight of light gas e.g., one pound of air is more easily handled by an ejector than one pound of water vapor.

What is the maximum discharge pressure of an ejector?

The maximum discharge pressure (MDP) is the highest discharge pressure that an ejector can achieve with the given amount of motive steam fluid passing through the motive nozzle. If the discharge pressure of an ejector exceeds the MDP, it will become unstable and break the operation. If this occurs, an abrupt increase in suction pressure happens. Since increasing the discharge pressure more than the MDP causes a loss of performance, it seems rational that reducing the discharge pressure below the MDP should have the opposite impact. If the compression ratio (discharge pressure to suction pressure) of the ejector is higher than 2:1 then it is called a critical ejector.

Calculation of flow rate of motive steam requirement:

The flow rate of motive fluid is a very crucial parameter for the ejector operation. It can be calculated by the following method. Other than this, it can be calculated in ASPEN Hysys software as well as by graphical method.

The following equation has been developed by the Heat Exchanger Institute,

Ejector Datasheet

The below figure (Fig. 5) shows a sample datasheet of an ejector.

Eductor vs Ejector: Differences between an Eductor and an Ejector

The terms educator and ejector are used interchangeably. Their working philosophy is similar, and both of them work based on Bernoulli’s principle. However, some engineers believe there is a slight difference between the eductor and the ejector. The difference between eductor and ejector is mentioned in the table (Table-2) below: