Following a basic manifold valve sequencing procedure when performing regular maintenance on a differential pressure (DP) measurement installation with transmitter is generally straight forward. Understanding the “why” however, is what most people don’t often think about. In this post, I’ll be providing suggestions on how to correctly operate manifold valves prior to zeroing a transmitter and outline why it’s important to stay consistent across different types of applications. Please note, since installation characteristics vary, always consult the product manual and site-specific safety documentation before zeroing a transmitter or performing any maintenance.
Zeroing a DP Transmitter with 3 and 5-Valve Manifolds
3-valve manifold schematic shown during typical operation
Recommended ProcedureWhen preparing to zero a transmitter, the first thing you should do is close the low side block valve. The reason we close the low side instead of the high side is mostly due to steam applications. If the high side was closed first it would leave a high pressure cavity inside of the manifold. Once the equalize valve is opened (step 2 of the procedure) it creates a potential for that high pressure cavity to push some of the low pressure wet leg out of the low side wet column. Although this would cause no damage to the transmitter, it could lead to further measurement issues. Once operational, the condensate would need to reform before the wet legs are back to their normal column height on each side. If the transmitter is zeroed prior to this occurring, the technician could see some very unexpected DP changes.
Once the low side valve is closed, you can then open the equalize valve. The process coming from the open high pressure side of the line will now move through the equalize passage and should create an equal pressure on your low side diaphragm ultimately netting a zero DP. You can now perform the zeroing function on your transmitter.
After zeroing the transmitter, you can close the equalize valve and reopen the low side isolate valve returning the transmitter into service.
The Importance of ConsistencyWhether you are measuring the DP of liquid, steam, high pressure or low pressure gaseous process, it is very important to be consistent in your overall valve operation. Oftentimes technicians believe they need to perform different procedures for the various applications mentioned earlier, all while still using a 3 or 5-valve manifold. In most cases, this isn’t true. Creating a similar habit when using the same type of manifold, even for various applications, will help ensure the quality and consistency of your measurement.
Click here to learn more about Emerson's Rosemount family of manifolds.
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