Recirculating sample Point

[SeanB]

I have to design a sample point to be located on a diesel rundown header. My client is requesting that this be a recirculating sample connection. Normally this would be easy to do around a control valve or some other source of pressure drop. However, this is on rundown piping where there isn't pressure drop source. For this reason I have been considering installing a restriction orifice and sampling around that. However, my rundown rates vary from about 450 GPM to 1780 GPM. I was going to size the RO for 1780 GPM with a dp of 10 psi. At my min flowrate then the dp would only be 0.5 psi. Is this enough pressure drop for recirculation?
Any comments or other suggestions would be greatly appreciated.

 

[sshep]

Is there anyway to recycle your sample to some lower pressure source upstream (even way upstream)- back to the tower feed pump suction for example? The small sample flow you need will be negligable recycle in the size of system you describe. A restriction orifice sounds like a poor alternative.

As a side note: Sampling around a control valve as specified by many designers is not generally desirable in my field experience. This is because you must have a product flow to get a sample (i.e. to see if you are on spec), and when wish to stop all forward flow you must also remember to block in the field sample bypass.

just a thought, sshep

 

[SeanB]

Maybe I need to describe the system a little better. Where I need to sample is located in the tank farm, a long way from any process units. This is "grab sample" point. The operator will attach his sample bottle and open the self closing valve and fill his bottle. Without a recirculating line the fluid upstream of the valve will be stagnant and will have to be flushed each time a sample is taken. This is what we want to avoid. So I propose installing a RO. My sample source supply would then flow around the RO from the higher pressure side to the lower. This way there will always be flow is the sample line - avoid the flush. does this sound reasonable?

Amother option I was considering was installing a small pump. Each time a sample was taken the operator would turn on this pump, flushing the line, and then take his sample.

 

[epoisses]

Sounds expensive to me to put a pump in just for that purpose and I guess you wouldn't want your line capacity limited by an RO either. Aren't there any "close to zero" stagnant volume sampling systems available for this purpose? You may want to contact Dopak or equivalent companies to see what they can do instead of inventing your own system.

 

[roker]

you can use an air operated pump or a peristaltic pump
roker

 

[quark]

Sampling valves used in purified water and WFI systems can be used in this case. They are just like flush bottom valves used for process reactor discharge but of smaller size.

You can also use zero dead leg valves(a three way type valve) which allow circulation and sampling as well. There will be total recirculation when the outlet port is closed.

Regards,

 

[m777182]

If there is compressed air available I would suggest a very simple sampling device: you need four valves (or two three- way pieces), hand or electromagnetically driven and a piece of pipe. On both ends of the pipe fasten the valves.(For understanding of the princip it is easier to use three way valves). The first end of the valve 1 is attached to your sample side. The other end 2 is where the operator comes with the bottle.Both third ends of the valves connect to compressed air.Now you start samplingpen the first valve direction sampling point-your pipeline.Open the second valve on operator' side direction compressed air and flush your line with air.Close valve 2 and alow the line to get filled with fresh sample.Close the valve 1 direction sampling point and open it to compressed air. Now you can take your new, fresh sample at the valve 2.It can be done with some time-relays automatically.
m777182