Pressure Measurement in Q&A-(Part 2)

Reviewing questions in part (1) here that are covering the famous techniques for pressure measurement, and we could continue ..

The following questions will cover the below illustrated topic (Accessories):

Q8. What are the famous accessories used with pressure measurement?

We have many accessories, but the famous ones are in the following figure:

Some of these accessories are mandatory to be used and others are preferred, all depending on the process parameters. Will have them one by one in the following questions, starting with the "valve manifold".

Q9. How to use the valve manifold with the pressure transmitter?

"Manifold" is simply a set of manual valves incorporated in a single device, used to control (isolate/equalize) the pressure from process to transmitter for maintenance/calibration purposes.

Will cover here the following 3 manifold types:

a. Single block/bleed manifold.

b. Three- valve manifold.

c. Five-Valve manifold.

The following illustrations indicate the 3 types:

Q10. When to use each type of the above 3 manifolds?

"Single block/bleed" manifold is used with a single point pressure measurement, while "Three- valve" & "Five-Valve" manifolds are used with differential pressure measurement.

"Five-Valve" manifold is to be used when it is needed to vent/drain the trapped pressure through a tube to a remote location.

Q11. How to isolate the pressure transmitter from the process for maintenance?

For "Single block/bleed" manifold .. This video is very descriptive:

For "Three- valve" manifold, To follow the following procedure:

1 - Shut high-pressure block valve.

2 - Open equalizing valve.

3 - Shut low-pressure block valve.

4 - Open bleed valve.

For "Five - valve" manifold, it will be the same sequence, but bleed valve will vent/drain the trapped pressure to a safe place away from the transmitter.

To reservice the transmitter the steps will be reversed from 4 --> 1.

It is always recommended to consult the transmitter/manifold vendor document for the best proper sequence.

Switching to the next accessories which are used for "Pressure Pulsation" Damping ..

Q12. How to mitigate the effect of pulsation on a pressure gauge?

Pulsation has a bad effect on the internal mechanical components of the gauge - Video example is here:

This issue could be mitigated using the following 2 techniques:

a. Filling the gauge with a viscous liquid.

The gauge is to be filled with a viscous liquid (Glycerin, Oil, .. etc), so the liquid inherent friction has a “shock-absorber” effect that absorbs the pulsations from external vibration.

Filled liquid also ensures a correct/stable pressure reading, besides cushioning and lubricating the internal mechanic/moving parts.

The following video indicates the whole technique compared to another vendor product (without filled liquid):

It is recommended to consult the vendor for the suitable fluid to be used (not just Glycerin or Oil). Proper liquid is to be selected based on the environmental temperature, as some liquids with specific temperature could harm the glass and cause a hard reading.

b. Using Snubber

Snubber is a piece of fitting that restrict the liquid flow passed to the pressure gauge. It reduces the gauge’s response rate and allows the gauge to reach the line pressure smoothly. Line pulsation is damped this way and the gauge is protected.

Switching to the next accessories which are "Pigtail / "U" siphon" ..

Q13. Why using "Pigtail Siphon" or "U Siphon"?

Usually they are used with steam lines. Steam high temperature could cause damage to the sensing element if directly connected. So "Pigtail Siphon" or "U Siphon" - which are simply “low” spot in the impulse line - are used to accumulate the condensed steam (water) and act as a liquid barrier that prevents the hot steam from reaching the pressure instrument - The following graph illustrates it:

Next is the "Diaphragm Seal" accessory ..

Q14. What is the "Diaphragm Seal" and why to be used?

Simply it is an "Isolating Diaphragm" used to seal between the process liquid and the instrument in case of harsh process material (Hot or Corrosive). Also it is used to avoid impulse line clogging and finally to avoid "Dead-End", where organic process fluid would stagnate in the tube and harbour microbial growths - The following video includes a quick preview:

Diaphragm seal could be used for local or remote sealing as per the following illustrations:

Some diaphragm-sealed pressure transmitters are equipped with close-coupled seals rather than remote seals. This is to minimize hydrostatic, temperature, and time delay effects caused by fill fluid inside the capillary tube - As per the following illustration:

Finally we have some other accessories .. Will talk about "Mounting brackets" & "Heated enclosure"

Transmitters are sometimes to be remotely mounted beside the pipeline/equipment. This way "mounting brackets" are needed to hold it over iron pipe.

For some cases - where the ambient temperature may become very cold - a protective measure against fluid freezing inside a pressure transmitter is to house the transmitter in an insulated, heated enclosure as per the following illustration:

Next part will continue with questions that cover the rest of the topics "Discrete pressure measurement" & "Calibration" ..