Happy new year, hope all the best for 2018.
On site we have a feed water pump discharging water through HP Economizer to HP Drum, though 30% and 100% control valves. At openings of 30% valve lower that 30%, the flow values is 0. The flow is measured with the use of an orifice and a differential pressure transmitter. The transmitter is OK, it was checked. Can you please suggest any solution?
Please, check the DP of transmitter at 30% & less than 30% opening. you will get an idea about the flow by seeing the DP at transmitter.
Assuming the two control valves are in parallel. At the 30% valve, One has to check and confirm the upstream and down stream manual isolation valves are full open. The valve stroke is physically verified with the displayed value on the Controller.
Another issue, which is very unlikely, the Orifice location on the Piping is as per the recommended Engineering Practices.
First, you need to take a look at the installation and see if you have the minimum requirements, such as outlet and inlet run, filled pipe, vertical installation with flow going up (the best way to install any flow meter).
Another possibility is to check is using a clamp on flow meter to measure the flow beside your DP flow, and then figure out what is happening. The clamp on should measure a similar flow comparing your DP flow.
> At openings of 30% valve lower that 30%, the flow values is 0.
I did not catch that the DP zero reading is only below 30%.
Check the low flow cut-off setting in both the DP transmitter and the control system.
30% flow and less translates into 9% and less of the d/p xmtr span.
typically, you generally require two properly ranged transmitters. also confirm that the square-root extraction via transmitter or DCS is properly coordinated.
I wonder if the flow measurement is realized with an orifice plate? It is not unusual that there is a flow cut off below 10% of flow when flow is measured by measuring differential pressure on orifice plate taps.
First thing you need to do is to check valves on delta P transmitter.
Open the equalizing valve and adjust/set zero of the transmitter. Then if there is an option to show or to measure differential P (to have readings on the display of the transmitter), you should use it.
This way you can see if your transmitter has correct readings of the differential pressure. The flow is usually coefficient * square root (differential pressure).
I have usually seen installations where regular delta P transmitter is used and square rooting and flow correction by pressure is done in DCS.
There were also a few installations where transmitter that was used could be configured to output mA signal proportional to the calculated flow.
Like I said above, first be sure that you have differential pressure readings. If not, then you have a problem in impulse tubes or transmitter valves. If you have readings of delta P when valve is open say 15%, then check if cut off option is active (on transmitter or in the DCS).