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]

Difficult to diagnose remotely.

Does the gas flow vary rapidly?

What are the lengths of pipe or tubing u/s and d/s of the regulator between larger volumes?

Buffer tanks before and after would help for sure, but how the flow varies is critical.

Are they any controls on the regulator to change its control parameters?

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

 

[Prometheus21]

I second LittleInch here, difficult to diagnose remotely.

That being said, in my experience (Europe), machines will give you error codes if given insufficient flow. It will however shut off at insufficient/exceeding set operating pressure.

1. Has this system worked before? From what you describe it seems that way. What changed?

"Even when I set the regulator to 32 psig the pressure can swing out of range shutting off the stripper" - Swinging what way? Downwards indicate leakage, lack of constant upstream pressure, rapidly flowing gas ++. Upwards indicate internal leakage in the pressure regulator (it doesn't regulate the pressure properly), or the pressure supply effect (increase in downstream pressure combined with decreasing cylinder pressure) - since you state the upstream pressure is constant you can disregard the last part.

2. Make sure that the pressure regulator can deliver sufficient flow at your set operating pressure. Too many people size their regulator based on it's maximum flow capacity (which at that point the pressure regulator has no function, it does not regulate the pressure).

 

[Latexman]

Is this a long time problem? Is this a recent problem? What changed? What remedies have you tried? When does it usually shutdown? Start-up, running steady, near shutdown? Does the shutdowns coincide with anything? Like full heat? We need data/info. from "boots on the ground", that's you.

I used to have a stripper that removed phenol from water before we sent the water to wastewater treatment.

Good Luck,
Latexman

 

[hacksaw]

Pressure regulators only work with resticted or zero flow downstream.

If your sripper operates at some pressure, its gas / vapor discharge requires a back pressure regulator compatable with the fluids involved.

 

[LittleInch]

Also post the data sheet of the regulator.

They work best when at about 40 to 70 % of flow range with the correct pressure drop.

That is quite a tight pressure range of your vessel is quite small and if the tubing is more than a metre long. You have about 5 ft/sec I think in your upstream tubing so shit 10ft/sec d/s.

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

 

[throwaway989]

@littleinch

I haven't done the velocity calc but it's a 2 scfm flow in a 1/2" tubing.

Upstream length is about 30 feet and downstream length is about 2 feet

There are no buffer tanks unfortunately.

Only control is changing the setting on the valve. Here is the particular valve here:

https://lowflowvalve.com/products/jr-series/

@prometheus21

1. This is a new install and it's a problem since we first turned it on

2. It swings upwards and downwards. If you look at the flow curve you can see it has a rather large droop for this needed operating pressure range:

https://lowflowvalve.com/products/jr-series/

@Latexman

It's a new install and it's been like this since we first turned it on. Usually the stripper will trip in the first minute or 2 starting up. Sometimes it will run for hours and then trip later on the same issue. it seems like such a sensitive system and I wish the operating range wasn't so tight

@pierreick

I will try to see if i can share a drawing

As for the tight operating range, I'm a bit limited in what I can say since it's considered a proprietary application. Not sure why but just an engineer.

@hacksaw

Not sure a backpressure regulator will work since the upstream pressure is pretty constant

 

[LittleInch]

Looks to me like a totally unsuitable control valve.

It specifically says its a very high pressure 4000psi, regulator. Why did anyone choose this regulator?

Do you have the specific valve data sheet for the one you have?,

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

 

[Compositepro]

The JR series literature that you linked covers a very broad variety of valves. There is one that is for low pressures like yours, but the spring ranges listed do not cover 32 psi. What is the exact model number of your valve? They look like expensive, high quality valves, but spring-loaded pressure regulators are generally not very precise. There is usually a rather wide dead-band between where they supply pressure and where they relieve pressure, and the pressure can drift between these two values. It depends on how well the internal valves actually seal. With these regulators you must always make the pressure adjustment the same way. Either increase the pressure to set point or decrease the setting to set point. Which way to use depends on whether the Supply valve or the relief valve seals tighter. This can change due to a particle of dust in the valve.

The only way to get around this issue is to use an instrument grade regulator. These contain a clever pneumatic amplifier that will operate the valves without the spring having to move at all, which is the cause of droop in the regulator. However, instrument regulators always vent a small flow to atmosphere.

 

[Prometheus21]

Yeah, that seems like a poor choice for a regulator IMO.

"If you look at the flow curve you can see it has a rather large droop for this needed operating pressure range" - Yeah. The lowest inlet pressure rated with the flow curves are 100 psi, you are operating at 59 psi. And with a set working pressure of 32 psi you will have a bad time.

Note that Cv is measured at the regulators fully open position, and does therefore not describe the overall performance of the regulator. At maximum flow capacity (Cv) the regulator can no longer control pressure. On the other end you have seat-load conditions; the regulator will seize and lockup due to flow/no-flow conditions. Both is a no go when selecting regulators.

You would benefit by using a low inlet pressure regulator (e.g. 10 bar (145 psi) inlet pressure MAX and setting it to operate at 2.1 bar (30 psi). These exists and are quite common in the medical industry. As Compositepro states: "However, instrument regulators always vent a small flow to atmosphere." Now if this is critical, and a no-go; then specialty laboratory regulators do exist (regulators for toxic gases for example).

 

[georgeverghese]

Causes my be :
a)The spring range you've got on this forward pressure regulator may be too wide for the operating range expected at this stripper, resulting in poor sensitivity.
b)The flow rate may be much lower than say 30% of full flow for this non pilot operated regulator.
c)Downstream stripper vapor holdup volume is small, and minor transients during stripper operation are too fast for this simple regulator to respond to - this may be related to (a) also.

Would say that for (a), if you want an operating range 30-34psig, the preferred spring range should be say no more than 25-40psig. What does the datasheet for this regulator say? Spring range going up to 50psig in this catalogue may be okay.

Velocity at 32psig is approx 8fps, and pressure drop is low on 1/2inch tubing, assuming this gas approximates physical props of air at 25degC. At 59psig, velocity would be lower.

 

[hacksaw]

OP:

"Not sure a backpressure regulator will work since the upstream pressure is pretty constant"

However, as you have explained the pressure regulator cannot control the pressure.

You can set the regulator but without downstream containment or partially restricted flow, it cannot maintain steady pressure.

 

[throwaway989]

@LittleInch

It's the 0.012 CV, 5-50 psig range. As you can see big droop

To be honest with you, this was part of a vendor package and I should have looked more into their work with greater detail.

@CompositePro

It's the 0.012 CV, 5-50 psig range. As you can see big droop and not too precise at all

@Prometheus21

That's some good info thank you.

Do you have any recommendations of companies that make such specialty valves?

@georgeverghese

"c)Downstream stripper vapor holdup volume is small, and minor transients during stripper operation are too fast for this simple regulator to respond to - this may be related to (a) also."

Could this also be the cause of the regulator outlet pressure swinging? The flowrate also swings with the outlet pressure swings (I know that's common sense but just wanted to state that).

Thank you everyone for all the help!

 

[LittleInch]

If you look at page 4 of the spec, you see at 25 psi, even with 100psi inlet, this valve just can't supply 120 scfh, (your 2scfm).

This is just totally the wrong valve. IMHO

To stand any chance of working I think you need the 0.2CV valve and even then its just designed for much higher inlet pressures.

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

 

[Prometheus21]

"Do you have any recommendations of companies that make such specialty valves?" - If it was the European market then I could easily arrange for the company I work for to send you one. To the US on the other hand? Way to much hassle for a single valve due to export/import regulations (in my experience). My best bet would be to reach out to your nearest Swagelok distributor, they have a multitude of regulators that is suitable based on your description. (Not an endorsement - just my personal opinion)

 

[Latexman]

I'll second Swagelok regulators. Take a look through their K series catalog. There is a wide selection. Surely something will work.

If you find you want to look at lower pressure / higher capacity regulators, take a look at Emerson-Fisher regulators.

Good Luck,
Latexman

 

[pierreick]

hi,
Don't know if this is feasible, can you consider a 2-stage process where you go from an initial pressure to an intermediate and then to the final pressure?? This should improve the sensitivity. Let say from 59 to 45 and 45 to 32 psi.

https://www.vfs-ltd.com/products/pressure-regulators/

Pierre

 

[georgeverghese]

Based on air at 25degC, I got a Cv of 0.07, but your gas props may be quite different.

So, its the controls interaction between this inlet regulator and the backpressure regulator on the stripper overheads exit that's making the stripper pressure swing out of control. At the very least, suggest the exit backpressure regulator be replaced with a pilot operated one. We know nothing about this exit regulator at the moment, and we dont know if the Cv selected for this inlet one is correct either. Droop curve for 100psig inlet at 0.08 Cv doesnt look good at 120scfh, 25psig set pressure, but would be directionally better with inlet pressure of 59psig, 32psig set pressure.

 

[georgeverghese]

The behaviour of the exit regulator may in turn be caused by some disturbance further downstream that results in a pressure spike or dip. Take a look at the whole system instead of just this inlet regulator and try to map out the sequence of events here that results in these inlet and exit regulators chasing each other's tails.

 

[1503-44]

I think I'm agreeing with George. Seems like a transient problem. Possibly worsened by what is controlling the on/off valve.

Why is an on/off valve feeding a regulator? I can't imagine that helping with the regulator's operation, but neither should it affect a properly working regulator, other than allowing the regulator to become active or not, then slugging it. Slugging it when the valve goes on is not going to help. I do not understand the on/off valve's purpose.

It would seem that all that is needed is a properly sized regulator. The on/off valve might be beneficial, if the regulator fails open, in order to shut off flow entirely, but that's seems to be a different problem.

I don't see that the regulator would be affected adversely by downstream pressure. It would only cause the regulator to work actively, if lower than the set point, or close entirely if above it. But I also do not understand if the regulator is controlling upstream, or downstream pressure, or what causes the on/off valve to open or close. You say it is controlling stripper pressure, so that apparently makes it a backpressure regulator. But then you close the valve feeding it when pressure is outside the range of 30 to 34 psig. If the regulator is controlling the backpressure to 30-34 psig, how is then that there appears to be a 59 psig pressure in there somewhere? What is keeping that at 59?

HOW ABOUT you make a little sketch showing these items and the diameter and lengths of the connecting tubing and where you are measuring the 59 psig pressures and the 30-34psig pressures. You have 19 posts without a diagram, or a solution, so it's obvious we do not understand something. Let's make sure it is not the arrangement and pressure points.

I think your on/off valve is slugging the regulator and your tubing pressure drop is too large and lagging, causing your 30-34psig press to go out of bounds, but hard to say, especially since I am still guessing what your system actually looks like and where that 59psig pressure is.


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