Sizing Control Valves-Outlet Pressure 6

[RJB32482]

Hello everyone,
When looking at sizing control valves, what outlet pressure should be used in the calculation? So lets say I have a valve that has an inlet nitrogen gas pressure of 120 psig. The outlet pressure used in the correlation should it be the expected pressure drop through the valve or the pressure far downstream of the valve? So lets say the source about 25 feet downstream of the valve and at atmospheric pressure normally. Should I used delta P of 120 psig?

Thanks.

 

[davefitz]

If the source is clean nitrogen, then the process conditions you described can be well met by a valve which will frictionally choke the flow, as opposed to acoustically choke the flow. An example of a valve that will frictionally choke the flow is the CCI Self-drag valve. Yuo would only need one valve in this case, not two, and there would be no noise issues, as might happen if you used a conventional valve that had acoustically choked flow.

 

[JimCasey]

This is not an application for a CCI Drag valve. Those are for 3 or 4 leagues more severe than this.

I wouldn't even bother staging this one. The pressure is not high, the flow is small, and your limitation on control will be the sensitivity of the pressure transducer.

 

[RJB32482]

So you are saying that the control valve can take the 120 pressure drop at 0-100% of total opening? We wouldn't need to drop the pressure before the valve?

Thanks.

 

[JimCasey]

Sorry, I got a phone call in the middle of that last post and had to bail before I got to explain.

Staged pressure reduction stations go back to when this was all done with regulators. Even before I was in the business. Regulators have, among their other characteristics, a property called Inverse Sympathetic Ratio. It's caused by the forces exerted on the plug by the process medium, compared to the forces exerted on the diaphragm by the sensed pressure. Nothing "Automatic" inside a regulator, just proportional force balance. 3% is not an unusual number for ISR. So if the upstream pressure increases by 10psi, the downstream pressure DECREASES by 0.3 psi. You'd be shut down. Or, of your N2 system goes a little soft by 10 psi, the outlet pressure of the regulator would INCREASE by 0.3 psi. That would be a HUGE swing for you in this application since your setpoint is only 0.1445 psi (4" WC). If you staged (2) 3% regulators, the outlet pressure swing would drop to 0.03^2 or less than a tenth of a percent. You wouldn't even SEE the output change due to swings in upstream system pressure unless you were watching verrry closely. So why don't you just use regulators? Regulators have other fun characteristics such as droop and high seat leakage but I won't go into all that here.

But you are not looking at regulators. You are looking at a control valve. With a control valve, the motive force comes from an independent compressed air system, with a PID controller telling the valve where to go. It looks at your downstream pressure, compares it to setpoint, and makes any corrective action necessary to keep it to zero error within the limits of sensitivity of the pressure transducer reading it. You only need a Cv of about 1, so the orifice is about 1/4". Not much force is required on an unbalanced plug to overcome 120 psi DP across a 1/4" hole. Your thumb will do that.

You can very easily take that kind of drop in one stage through one valve and control accurately. I listed a number of valves in an earlier post that would do just that.

 

[RJB32482]

The valve will not be controlled by pressure. The purpose of this control valve will be to add nitrogen to a pipe if the LEL gets too high in the header. The header has a baghouse that can only take 8 " WC. So I will have relief in the header at 6" WC. So as LEL gets ligher, the valve will open and vice versa. So my question will I be able to keep the outlet pressure of the valve at 4"WC with 100 psig inlet pressure (at 0-100% open) to the valve providing 200 SCFM at 50% open? Or would I need a control valve with a regulator downstream to make sure I have only 4" WC at full open to full close valve positionings?

Thanks.

 

[JimCasey]

We can re-scale the valve so you get 200 scfm N2 at 50%. So you need a Cv of about 8. I don't know why you want to limit the valve to 50% but I trust you have your reasons. It appears if you want to control on the other points we evaluated earlier that you are compromising resolution.

200SCFM will flow if the Cv is 2.56. So just give the valve a signal that will not cause it to open to a Cv of greater than 2.56,(look at the specific curve for the valve you are considering) and you won't exceed 200 scfm.

Note that if you direct the flow directly at the bags it will probably do the same thing as the pulsers and knock the dust cake loose, so you might want to arrange a sparger ring inside the chamber to distribute the N2 uniformly.

An you still don't need two stages.

 

[RJB32482]

The reason I want the valve to get 200 SCFM at 50% is for a safety margin in case the LEL gets even higher in the system.

So I'm new at control valves so if I size a control valve at 50% stroke and 120 PSIG pressure drop for a Cv of 2.56, the Cv will increase as the valve opeens to 75%. Now will the delta P stay at 120 PSIG or will the outlet pressure of the valve increase? So does a higher Cv just mean a higher flow or can the pressure increase also (getting above 4" WC and opening the relief valve)?

Thanks for the learning.

 

[TD2K]

As you open the valve from 50% to 75% the flow through the valve will increase.

Will the downstream pressure increase? Depends on the hydraulics in the downstream system. If the capacity of the system is 'large' compared to the additional flow, the rise in pressure will be minor (there will be a rise in pressure since more flow needs a higher dP but the effect might be minor).

Now, in your case, if the downstream pressure is 4" WC and that's the pressure needed to flow through the ducting/piping and bag filters to the atmosphere and you increase (let's say for discussion purposes) the flow by 50% by going from 50% open to 75% open, you need about 2x dP. In other words, the 4" WC will increase to 8" WC or it would if you didn't have a relief device set at 6" WC.

 

[RJB32482]

So how would I size a control valve to make sure I never exceeded 4" WC but still was able to get at minimum 200 SCFM nitrogen into the system (not staging the pressure drop)? Would I have to size the control valve to have a maximum of 200 SCFM at around 80-90 % open?

Thanks.

 

[JimCasey]

1. Staged pressure drop is unrelated to, well, much of anything if you are not going to monitor and control the downstream pressure.

2. Exceeding 4" WC is, as TD2K stated, dependent on whether your baghouse can accept an extra 200scfm. I can't imagine that there would be a problem because 200 scfm into a baghouse is a tiny fraction of the what I estimate to be the normal flow, the bags are open to atmosphere, and if nothing else the N2 will flow upstream. If you completely clog the bags and you have a High LEL, lifting the vent in the baghouse is not your primary problem.

If 200 scfm is truly your maximum flow, then buy a valve that will be wide open at slightly above 200 scfm. You had other operating points, and this will give you the best resolution for your"normal"operation. Nitrogen ain't free, and controling it accurately will pay off in N2 savings as well as your primary purpose, safety.

 

[TD2K]

How do you prevent the downstream pressure from exceeding 4" WC?

That's the job of the pressure transmitter controlling the control valve. As the downstream pressure (assuming that is what you are controlling) varies, the controller readjusts the opening on the control valve to return the pressure to its set-point. The valve does not directly control the downstream pressure.