4 replies to this topic

[kowweihao]

Hi, 

 

Recently in my plant, we have been experiencing irregular flow meter readings when charging in a particular chemical. My production is in batches, so at every batch, we would charge in a fixed amount of chemical into the reactor by calculating the flow rate of the chemical through a pipe.

 

Normally, we'd see the readings increase at a steady rate i.e. 1 kg every 3 seconds until the set point is reached. The pump and valve closes at Set value of 75 kg, and the calculated weight charged in would be 75 kg. However recently, i have noticed that there is a slight spike in weight at the end of the chemical charging once the pump and valve closes. Sometimes the calculated weight would jump 1 - 2 kg additionally upon closure of valve.

 

My flowmeter configuration is the flowmeter, with 2 isolation valves directly before and after this meter. Recently the isolation valve just downstream of the flowmeter has been throttled to address an issue, and it seems like the flowmeter reading trouble only started with the throttling of the isolation valve. 

 

Long story short, i just wanted to know if throttling of a valve just downstream of a flowmeter could affect the accuracy of the readings? Especially during pump stop / outlet valve closure period.

[breizh]

Hi ,

 

Why do you need to throttle the valve ? An isolation valve should work fully close or fully open .

If it was working before , there no reason to change.

 

What is the accuracy of your meter ? What is the material ?

 

my 2 cents

 

Breizh

Edited by breizh, 03 February 2015 - 09:17 PM.

[Zauberberg]

Throttling an isolation valve upstream or downstream of a flow meter will definitely affect readings, because the flow pattern changes dramatically once a high-turbulence area is created in vicinity of the flow element. Throttling of the valve will cause flow path distortion, create swirls etc. which all affect the meter performance. Each type of flow meter (i.e. orifice, turbine, positive displacement etc.) have certain recommendations with regards to minimum straight pipe runs upstream and downstream of the meter. This is for the reason to have the flow as stable as possible through the measurement device.

 

Those two isolation valves are provided only for isolating the meter and taking it out of service for maintenance, not for adjustment of the flow.

[MTumack]

I agree with Zauberberg, throttling with those valves with make your flow considerably more chaotic and turbulent. The more turbulent your flow, the greater error you will see with your Flow metering.

 

The recommended Rule of Thumb is that most meters see 15(ID pipe) upstream of unrestricted straight pipe and another 5(ID Pipe) on the downstream side to prevent exactly this.

[kowweihao]

Hi all,

 

thank you for your all your feedback. The valve was throttled to reduce the flow rate as flow when fully opened was beyond the range of the flowmeter, and we were unable to get an accurate weight of chemical to be charged in. 

 

I will take your advice on the uninterrupted straight pipe length before and after the flowmeter, and discuss with the flowmeter vendor.