Level Transmitter and Level Gauge Design Tips

 Level Transmitter and Level Gauge Design Tips

Level / Vessel Sketches

What is this ?

Level sketch or vessel sketch is the drawing prepared by Instrumentation engineer to show various Level instruments nozzle details (Flange size, rating) on vessels / drums, their elevations which is used as input for Mechanical – Static (Vessel) Engineers for further engineering of vessel / drums.

Why this is prepared ?

This is prepared as an input document to Mechanical – Static (Vessel) department so that instrumentation interface is clear and documented.

Who benefits ?

It benefits both department engineers as interface details are documented and any changes are tracked as document will be revised.

Disadvantages / Advantages ?

Disadvantages :

For smaller project, if no of equipment are very less (such as 1 or 2) preparing this document would consume more efforts compared to large scale projects.

Advantages :

The interface between two disciplines is streamlined and there is no scope for difference in inputs and information. The changes will always be traceable. If ambiguity is found, this can be documented and corrected.

What are basic inputs required ?

• P&ID (Design level),
• IPDS (Design level),
• Mechanical data sheet (Design level),
• Piping Specification (Design level),
• Instrumentation Design Basis (Design level),
• Client / Process Licensor Specific requirements

What are different types of vessels ?

What can be different type of instruments put on vessels?

• Level Transmitter (Contact / Non-Contact type)
• Level – Differential Pressure Transmitter
• Level Gauge (Magnetic, Transparent, Reflex)

Different combinations you can have on vessel (Stand pipe / direct nozzle)

The Level Instrument nozzle can be directly installed on vessel. 

Figure 1 – Level Transmitter and Level Gauge Directly Mounted on equipment

Stand pipes are used for installing the level instruments.

Figure 2 – Level Transmitter and Level Gauge Mounted on Stand Pipe

It is generally based on no of instruments to be put on single vessel as it is not advisable to puncture vessels for many instrument nozzles. This will be governed by project specifications, process requirements and client guidelines.

Standpipes are generally used in following scenarios –

Multiple Level Gauges

More than 3 no instruments on same equipment

Project specifications

What all information to show ?

Equipment Details –

Vessel Tag no.

Nozzle Nos,

Elevations (in mm preferable),

Tank Lines (Lower & Upper)

Zero elevation line (either from TL or at other reference point),

Piping Specification, material 

Process Details –

Process fluid,

Specific Gravity or Density,

Alarm Levels (Low, Low Low, High, High High)

Process & design conditions (Pressure & Temperature),

Instrument Details –

Instrument tag no,

Type of instruments (Radar, DP Level Transmitter, Magnetic Level Gauge),

C – C (Center to center) distance in mm preferably,

Isolation Valves and sizes,

Drain & Vent valves and sizes,

Scope Breaks (Instrumentation, Piping, Static),

Traced Lines (Steam / Electrical)/ Jacket Lines,

Instrument Nozzle size, connection, rating, Elevations in mm preferably,

Stand Pipe, size and connection details

What not to show

Other than Level Instrumentation Nozzles, other instruments nozzle such as Pressure / Temperature, Inlet, outlet nozzles, Nozzle orientation

Example: Single Level Transmitter & Single Level Gauge on a Vessel

Single Level Transmitter & Single Level Gauge

Service – Water

HLL = 1400 mm

HHLL = 1200 mm

LLL = 350 mm

LLLL = 300 mm

Density – 1000 kg/m3, Specific gravity – 1

Low Low Level should be at 10% of Transmitter Range (Subject to project specifications)

High High Level Should be at 90% of Transmitter Range (Subject to project specifications)

So based on the above guideline, different combinations can be tried to finalize the Transmitter Nozzle elevations.

Bottom Elevation (HP Tap) : 150 mm       (Refer Tip in the above diagram)

Top Elevation (LP Tap) : 1550 mm

Hence C-C Distance : 1400 mm

Transmitter Range = (HP Tap Elevation – LP Tap Elevation) * Sp Gravity

Transmitter Range =  (1550 mm -150 mm) * 1     = 1400 mm

Hence Select Transmitter Range : 0 – 1400 mm

Calculate LL Level of Level Transmitter

With This, LL level will be at @ 10.7% i.e.

LL Level of LT =     [(300 mm – 150mm) / 1400 mm] * 100 = 10.71%

Calculate HH Level of Level Transmitter

And HH Level will be at @ 89.28%. i.e.

HH Level of LT =    [(1400 mm – 150mm) / 1400 mm] * 100 = 89.28%

Level Gauge Nozzle elevations can be at same elevation as it covers full transmitter Range.

Tip – The bottom nozzle elevation is at 150 mm due to welding at TL point of vessel, wherein welding thickness is @ 80 – 100 mm, Nozzle centre is at 150 mm and nozzle size is generally 2” for Transmitters due to which Instrument nozzle welding may clash with Vessel shell welding joints. Hence it is recommended to use 150 mm as general guidelines for bottom elevations.

In case it is required to go below 150 mm due to Low level alarm values, first discuss with process group if the alarm value can be changed and shifted towards upward side or discuss with client if LL alarm value at below 10% is acceptable or discuss with vessel engineer / group if nozzle elevation below 150 mm is possible.

Article Written by :

Nilima singh

Instrumentation and control Engineer

13 year experience in Detail engineering

Worked on various Refinery, Oil & Gas, Petrochemical and chemical projects