Dear All

The signal from flow transmitters which are installed on LPG tank discharge lines is not steady. it is bumping and these signals are wired to ESD system. the signal value even going below and above 4-20 mA (less than 2.5 mA and more than 25.5 mA). Due to this the ESD latches Bad PV value and only after resetting, the value returns to normal and also in the graphics. please provide your valuable suggestions to resolve this issue.

Thanks and regards

syed Hashmi
[email protected]

 

syed,

Sorry, but you have not provided enough information for anyone to help you. Some of the information that might help is:

1. What flow measurement technology are you using?

2. How many transmitters are affected and how are they related?

3. Is this a problem for a new installation or an existing installation? If existing, what has changed lately?

4. Is this installation directly downstream or upstream of a pump? Have you looked a pressure gauge directly upstream and downstream of the flowmeter?

5. Does this happen all the time, periodic, random? During a storm?

6. Do you have any EMI sources near by?

7. Is your shield grounded at only one end?

8. How long is your 4-20 ma loop?

9. What have you done to troubleshoot it? Broke the 4-20 ma loop and used a process calibrator as the receiving device to read the flow directly from the transmitter, checked the power supply, looked at the output with a scope, replaced the transmitters, etc.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP
Any information is provided on Caveat Emptor basis.

 

Dear hasmi,

There are rules that have to be strictly followed to locate any flow measurement equipment if you wish to get high accuracy of the measured flow. Ideally it has to be placed a a straight section of of the flow conduit.

The straight conduit upstream to the equipment shall be at least 15 X diameter of the conduit. The straight conduit for the downstream has to be at least 5 X diameter of the conduit. There should be no flow restriction element between these two sections except the one that given by the equipment itself.

 

LPG may be gassing off. This would account for significant magnitude signal errors.

If this is an occasional transient condition then you need to be able to detect this and have the means to freeze the control settings so the control does lose the plot.

For example, some density meters have an alarm that detects gas pockets from the onset of a step change in the signal and then they can transmit the last good value until either the transient is passed when they can revert to live readings, or they will time out and default to the live readings with an active alarm.

Amongst other things, you may need to review the installation, not just to see if the meter is installed correctly but also to see if the discharge is drawn from an appropriate position in the tank and does not cause vortexing which will draw in gas, and that the system has adequate back pressure. Also make sure your flow is within the meter's best flow rate range. 2/3 max flow is often a good guide.

 

PS, of it is not a transient but a persistent condition you need to correct the problem, not manage it.