4 replies to this topic

[cat]

Dear all,

I have tried to countercheck the flow meter in the line through the use of sight glass.

Please refer to the attached file for the procedures conducted and observations during the test.

My problem is:

The flow drops down whenever i switch line from main supply to "sight glass" line and level in the sight glass does not go down. I am suspecting that no nitrogen was flowing or the bubble i had observed could have created a space at the upstream line of the sight glass (b/n nitrogen supply line and sight glass) and prevents the flow.

But the sighglass is located at considerable altitude as compared to the pump (~1m).

Previous tests have been successfully conducted before (~ 2-3 years ago).

Thanks for your insights.

Attached Files

•  sg.doc   1.38MB   21 downloads

Edited by cat, 22 February 2011 - 01:18 AM.

[Art Montemayor]

Cat:

Your description of what you are doing and your procedure both do not read well and are not easy to understand or interpret. Correct me if I am wrong if I state:

• You want to check the pumping capacity of a positive displacement pump by using a visible sight glass as a suction source for the pump.
• The testing of the pump’s capacity will take place simultaneously while the pump is working and on-stream.
• You first allow suction liquid to fill the sight glass while the pump is pumping. This takes place because your normal pump suction is under pressure and you can vent the top of the sight glass.
• Once the sight glass is filled to the level you want, you close the vent and block the sight glass outlet; then you pressurize the sight glass with nitrogen.
• To test the pump, you simultaneously block the normal suction and open the sight glass outlet that leads to the pump suction. The pump should continue to pump and you measure the time it takes to lower the liquid in the sight glass a given height (or volume).

Is this description correct?

If so, then what you are doing should work – assuming that valves are opening and closing as expected. If liquid is not being pumped out of the sight glass - BUT liquid IS being pumped - then the answer is that valve #5 is not opening and imposing nitrogen pressure on the liquid in the sight glass. Valve #2 two must be leaking also because otherwise, there would be no liquid being pumped.

However, you don’t tell us if there is any liquid being pumped or not. This is an old and tested method to test the pumping capacity of positive displacement pumps and I have used it numerous times on TEG positive displacement pumps on natural gas dehydration in order to verify and confirm the TEG recirculation rate. I know it works and that it is reliable. But the valves have to work.

[pavanayi]

Cat,
You also might want to check the NRV in the nitrogen line.

Also, did you observe bubbling each time you 'simultaneously closed #2 and opened #3'?
Did you repeat the same steps at any point, but starting by filling upto say 500 cc and not fully?

If valve 2 was passing, and nitrogen valve #5 and its NRV was working, with the nitrogen pressure of 690 Kpag, and line pressure of 220 Kpag, I expect the liquid inside the SG to be pushed down by the gas as soon as you open #3 (the bubbling??)

Edited by pavanayi, 22 February 2011 - 03:17 PM.

[cat]

@ Art Montemayor:

Yes sir. The description was correct.

Under the first trial, liquid was still being pumped even though valve 2 was closed, and this lasted for more than 5 minutes, but the level in the sight glass was not going down. We had suspected that valve 2 was passing, therefore we closed valve 1. DCS displayed that the flow dropped down to 0.2 kg/hr.

We conducted another trial, this time, we simultaneously opened valve 3 and closed valve 1 instead of valve 2. However, we did not test this set-up for a few minutes. After fully closing valve 1, we also closed valve 2, and similarly the flow dropped to about 0.3 kg/hr.

@ pavanayi:

By NRV, does it refer to the check valve?

I have seen the bubbling only once since we were not able to repeat the test because we can’t get the fluid out of the sight glass. Initially, we planned out filling up the sight glass to 700 cc as you can see in the picture; however, it was filled up totally when we simultaneously closed 2 and opened 3.

[pavanayi]

Cat,
NRV --> non return valve (the check valve). Apologies for using abbreviations without making it clear.

if all valves and check valves are working, the scheme should work!!. My plant checks calibration of 15 chemical dosing pumps each shift, so I know it works!!!

When you say "it was filled up totally", do you mean to say that you cannot see a level in the glass? and suspect it has overfilled? I am assuming here that the liquid is colourless as seen in the picture.

You might also want to check if there is fluid inside the sight glass in the first place (to empty it out). If what I described earlier has happened, all the liquid would have been pushed out by the high pressure nitrogen gas (bubbling), and then giving the impression of "filling up totally", when actually it was emptied out?

keep #3 and #4 open again, sufficiently long so as to ensure there is stuff inside the sight glass, then try again.
Still, if you say you cant empty out the sight glass, try instead to switch over to the standby pump ( I assume you have one), isolate the above pump and drain the suction line safely. There should be facility to do it. That should empty out the glass!!.

PS: I hope you have cleaned the sight glass before use, if you are doing this after a 2-3 year gap. There might be dirt that got stuck in the pipes/valves, blocking some constriction in the flow. You never know what all material find their way into pipes.