This article shall be applied for process engineers’ preparation of basic requirements to enable the piping engineers to finalize LC-LG Arrangements. LC-LG arrangements should be prepared through engineers in process engineering, control engineering, piping engineering and mechanical engineering departments close collaboration. Total co-ordination should be executed by piping engineer.
Work Step of LC-LG Arrangement
Piping Engineer should prepare datasheet based on the skeleton drawings. The following items shall be filled.:
- ① Vessel Item Number
- ② Service
- ③ Instrument Tag Number
- ④ P&ID Numbers
- ⑤ Dimensions of Equipment
(Prepare a sketch of Equipment outline, liquid levels) Then, the datasheet shall be sent to the process department.
Process condition shall be filled in, and visible range/control range shall be indicated by Process Engineer
- ① Design Pressure & Temperature
- ② Operating Pressure & Temperature
- ③ Fluid Name
- ④ Operating Specific Gravity
- ⑤ Visible range
- ⑥ Control range, if required
Then, the data sheet shall be sent to the control engineering department.
Instrumentation Engineer will select LC type according to the detailed engineering design data of instrumentation, and the following data shall be filled in.
- ① Quantity of level instrument
- ② Type of level instrument
- ③ Length (Center to center, Visible range)
- ④ Connection Size / Rating
- ⑤ Line Class
Then, the datasheet shall be sent to piping department.
Now piping engineers will select LG type based on process condition, and the following data shall be filled in.
- ① Quantity of Level Gauge
- ② Type of Level Gauge
Decide the nozzle height and valve assembly. Such information shall be indicated on the datasheet.
Then, the data sheet shall be distributed to the disciplines concerned.
- Mechanical Engineer will check interference between the nozzle welding line and equipment welding line. If any, nozzle height shall be adjusted by the piping engineer.
- Finally, LC-LG arrangement shall be followed to P&ID by Control/Instrumentation Department
LC-LG Design Criteria
- The fluid name shall be specified.
- Polymerization, fouling, slurry, and other data required for the selection of LCs and LGs which are not indicated on P&IDs shall be entered in the process data sheets.
- In the case of multi-phase fluid, the specific gravity of the individual fluid shall be entered.
Level Control (LC)
The typical arrangements of level instruments are indicated below in Figure 1.
The external covered displacement type or deferential pressure type level instrument should be selected by the following basis:
- Visible length over 1200 mm(4FT) : Deferential Pressure Type
- Differential pressure of differential pressure type level instrument should be recommended over 500 mm H2O. (Measuring range ≧ 500/Sp Gr)
- Diaphragm seal type is recommended for corrosive fluid.
- The external displacement type shall not be used for flashing services or averted external radiation services.
If displacement types are to be used, their installation types shall be entered according to the fluid properties and the client’s instructions, etc.. Normally, side-side(“SS”) LCs shall be used.
Connection Type of Displacement Level Instruments
Level Switch (LS)
- LS and LC should be installed on the same standpipe. However, LS and LC shall be installed separately for cloggy services.
- Type of LS should basically be external ball float or displacement type.
Level Gage (LG)
- The visible range of LG shall cover the control range of LC.
- In the following services, through vision type gage glass should be used.
- Liquid-liquid interface
- Fluid under 20° API
- Crude or residue oil
- Fluid include gum or sediments
- If measuring range is large for covering with one LG, two or more LG shall be used. However, the visible range of each LG shall be overlaid as follows:
- If the plural numbers of LGs are needed, the standpipe can be used. Standard size of standpipe connection should be 2 inch and the maximum length of standpipe shall be 4 m. If the standpipe length is over 4 m, standpipe size should be increased.
- In the case of standpipe connection and Low Liquid Level (LLL) is very close to the vessel welding line,LG can be connected at the same level of horizontal portion of standpipe.
- Connections to vessel -Level instrument connections shall be made directly to vessels and not to process flow lines or nozzles (continuous or intermittent) unless the fluid velocity in the line is less than 0.6 m/sec (API Recommended Practice 551). Connections and interconnecting piping should be installed so that no pockets or traps can occur.
Level Instruments Mounted Direct to Vessel
The typical arrangements of the level instruments to be mounted directly to the vessel are as follows:
Level Instruments on Standpipe: Typical Arrangement-
Recommended LC LG Arrangement
The typical LC and LG arrangements are shown in the figures below: