Correct DP Transmitter Isolation Procedure


Thread Starter


Hello everyone,

In the past I have been told that the correct way of using the valve manifold for differential pressure transmitters is to firstly open the equalisation valve before isolating the HP and LP line from the system.

We were told to do this because, if an instrument has been labelled wrongly etc and someone is fast enough in the control room, you can be contacted to reinstate the transmitter by closing a valve. If you isolate and lock off the tapping lines then it takes much more time to do this and if the site goes down it costs a lot to get it back up and running again.

I have been told however after that this could damage the transmitter, but I do not see how. I have created a pressure equilibrium and if anything protected the instrument more. The only thing that I can see happening is in certain circumstances liquid could fill the LP leg from the HP leg which would have to be drained before commissioning.

What do you think? I want to get my facts right before writing a procedure. I would also like to understand how I can damage the instrument by using my method!

Thanks :)
>firstly open the equalization valve before isolating the HP and LP line from the system <snip> I have created a pressure equilibrium

Not true.

When the high and low side isolation valves are still open, the differential pressure will create flow path from high side to low side through the open equalizing valve. It's the Laws of physics.

You only create an equilibrium when you have prevented the process from maintaining a continuing differential by closing the isolation valves. At that point, when the isolation valves are closed and the equalizing valve is opened - the pressure across the high and low ports equalizes.

There are different considerations on this topic. I learned from a steam guy and he wouldn't ever allow flow through the manifold and impulse lines by leaving the equalizing valve open before closing the isolating valves, as Mr. Spitzer explains in this article, along with other considerations:

The conventional view is here:

Many of the manufacturers of flow element manufacturers state procedures for commissioning, service or calibration. I saw one recently in the Rosemount annubar manual (starts on page 71) Rosemount Documents/00809-0100-4809.pdf

or "Manifold Operation" in the Rosemount 3051 DP manual, here (starts on page 41 pdf) rosemount documents/00809-0100-4001.pdf

Note it is not absolutely necessary to close both (H and L) isolation valves before opening the equalizing valve (E).
Thanks for the info guys. sorry for lack of reply. i'm the only C&I at this place so it's been difficult to get any time!
>> firstly open the equalization valve before isolating the HP and LP
>> line from the system <snip> I have created a pressure equilibrium
> Not true.

It is absolutely true if you are dealing with conventional process media. When the blocks valves are open and the equalizing valve is opened, fluid pressure from HP side will go to LP side as HP side pressure is higher than LP side and transmitter must show zero as dP=HP-LP. A pressure equilibrium is created.If the Tx doesn't show zero then it needs to be zeroed.

This won't work only when you are dealing with two phase fluids (both steam and water present) or anywhere,when the primary sensing element is designed for producing very low range dP, generally in gas or air flow measurements where the dP might be <60mmWC.

But you need to be very careful when dealing with high pressure and temperature, example in steam applications or with any sort of corrosive fluids. The fill fluid inside the impulse line should never be drained during live calibration.
One thing that wasn't mentioned, is it flow or level.

With steam flow if you open the equalization valve the resulting flow will upset the level in the condensate pots, it could take several minutes to recover. Not to mention the stress put on the tubing as it goes from ambient to steam temperature.

Another example a DP cell measuring the level on a condensate tank with a wet leg on the LP side. If you open the equalization valve the condensate will run from LP to HP side dropping the level in the wet leg, it could easily take half an hour to build back up again.

I have always closed one leg, opened the zero valve then closed the other.