Fluctuating Pressure Downstream of a Regulator

[throwaway989]

Hi all,

I have this small stripper at my plant that requires an operating pressure between 30-34 psig. I can’t get into the exact process but basically if it’s outside of that range it shutdowns (on/off valve closes).

It’s not run continuously only about 2 hours a day. When it does run the on/off valve opens (located upstream of the regulator) and about 2 scfm of gas flows through a small self containing spring regulator.

The upstream pressure is very constant at 59 psig. It's a 1/2" tubing.

Even when I set the regulator to 32 psig the pressure can swing out of range shutting off the stripper

Normally I would install a modulating control valve on an application but since it’s so small including the line size it doesn’t seem feasible

Any recommendations on what to do?

Should I install a pressure buffer tank ? If so any advice on how to size it ?

Pilot operating valve? Issues with the tubing layout?

Thanks you

 

[LittleInch]

Mr 44,

I agree we don't have the full picture as usual, but it is pretty clear from the data supplied that the regulating valve is too small / too low a CV and designed to regulate much higher pressures. George calculated a CV of about 0.08, versus the 0.012 fitted. The 59 psi is the gas feed pressure into the regulator.

The only way I can see this valve could ever work is that the flow is a lot less than the quoted 2scfm. Bearing in mind we don't know what is controlling this pressure or flow on the other side of the reactor.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
We know very little, neither the gas used for the stripping nor the temperature. How can we calculate a CV?
At some stage, before I deleted my reply, I requested a PID. Seems confidential.
May I ask what are the feedback from the contractor who did the work and the supplier of the PRV?
Pierre

 

[1503-44]

What's confidential about a on/off valve and regulator? We don't need the PID of the whole plant.

Yeah. Its the wrong valve. Thats pretty clear. That may be the beginning and end of the problem.

But, just trying to understand the equipment string, 59 at regulator inlet. So then the regulator is upstream of the stripper operating at 30-34? But then its a downstream pressure regulator. Or is it? And where's the On/Off valve? Why is it even needed?

Alpha-Numeric PID
[Upstream] ---> 59 psig ---> [on/off valve]----->[PCV]---30-34 psig--(Stripper)----> [Downstream]
Please Rearrange as necessary

Maybe even better, i'll just go eat lunch.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Looks about right to me.

We know nothing about how the reactor pressure is maintained for flow out, but it appears to be sensitive to gas flow pressure in.

Maybe the two are fighting against each other - dunno

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

I think they are. The confusion in upstream and downstream is enough to make that theoretically possible. The regulator is going open trying to reduce 59 to 32+/-. If its an upstream regulator and supply is enough to hold at 59psig, even when open full, then with a big Cv, the pressure at the oulet is almost the same as the inlet, so won't ever drop to 32, so the valve remains full open. If it is to work as an upstream pressure regulator at all, then the upstream pressure must be droppable by opening the valve. If the available supply is so great that no or very little pressure drop happens in the upstream piping, it won't work. It must be possible to evacuate the upstream piping at a greater flow rate through the valve and tubing then that pipe can be filled.

If a downstream regulator, then it stands a chance to work, but apparently will not, because its large size is letting too much gas through even when it opens just a little, 34 goes too high and it shuts off.

Conversely, if the valve was too small, then downstream pressure might drop to less than 32psig, as its outflow could be more than the inflow from the valve.

If downstream pressure sink goes above 34, then the valve also closes. The downstream pipe must also be capable of at least an equal outflow rate than the inflow rate the valve is passing into it.

We cannot just put in a valve with Cv of X and expect everything to work as we want and the charactetistics of the supply and sink must also be compliant.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TiCl4]

To add my $0.02 here, we also don't know what the speed of signal change is either. If OP changes the regulator to a properly-sized valve for the required flow, it still may be that the piping configuration and vapor holdup volume downstream (?) of the regulator is too small to allow sufficient time for the regulator to respond in time. An intermediate air receiver may be necessary to elicit proper control.

The air receiver would also be of use if there is another downstream PCV that is trying to control stripper pressure.

 

[LittleInch]

Quite lot of "known unknowns" here alright.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[throwaway989]

@1503-44

The purpose of the On/Off valve is to function as an interlock and to send gas to the stripper for 2 hours a day.

So once a day, the On/Off valve opens up to move gas into the stripper. After 2 hours if closes.

During the 2 hours if there are any interlocks such as the pressure going out of range it closes

"Alpha-Numeric PID
[Upstream] ---> 59 psig ---> [on/off valve]----->[PCV]---30-34 psig--(Stripper)----> [Downstream]
Please Rearrange as necessary"

Yep this is the correct arrangement!

The distance between the pressure gauge that shows 59 psig and On/Off valve is 1 foot.

Distance between On/off valve and PCV is 1 foot.

Distance between PCV and Stripper is 3 feet. There is also an elbow 6" after the PCV!

I agree that the piping is not optimal. It's too short with little straight pipe between the equipment.

Sorry all for the late replies I been traveling.

Thank you all for the help!!

 

[georgeverghese]

If the stripper feed gas is close to condensing at 32psig, the entire feed line to the stripper should be free draining. Else you'd get intermittent gas and liquid slugs from low point piping that may overwhelm the stripper unit and the exit overheads regulator.

 

[LittleInch]

throwaway989,

If you want anything more then you need to address the questions being raised.

I think most people see the current valve as being too small a CV / inadequate for your proposed usage and flow.

If you can't post a P&ID, can you sketch up how exactly this stripper works in terms of pressure and flow. If you're feeding gas into it at 32 psig for two hours there must then be somewhere for the resultant fluid to go. How is that flow and pressure maintained in the reactor? Another valve? An orifice? Something else?

Do you measure gas flow anywhere?
Is there a trace of flow over time so that we can see how steady or unsteady this is?
Same with pressure - how does it actually vary over time?




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.