Pressure Control Valve vs. Pressure Regulator 1

[Ashereng]

Hi Everyone,

I have a question regarding the interchangeability of a pressure control loop setup (pressure element, transmitter, PLC, globe valve) versus a self-contained pressure reducer/regulator.

My basis is a hypothetical case as follows:
1) 2" line, 300# ANSI flanges
2) sweet hydrocarbon gas
3) inlet pressure of 435psi reduced down to 100 psi outlet pressure
4) temperature ranges -20°F to 120°F

In general, I tend to use a pressure control loop setup on "process" lines, but happily use the self-contained pressure reducer/regulator on utility and service lines. However, I don't see why the pressure reducer/reguator can not also be used on process lines.

Any thoughts on this?

 

[TBP]

If there are a range of set points for several operating conditions, or other requirements that can vary on a regular basis, then a valve that can be adjusted automatically, or from a control room is the way to go, especially if it's located in a place that's awkward to access. There are other types of applications - like deaerator PRVs in boiler rooms - they pretty much just sit there maintaining whatever operating pressure has been chosen, often for years on end. If there's ever a change to be made, then the shift engineer gets a step ladder, and an 8" adjustable wrench, and makes the adjustment.

 

[jsummerfield]

Self-contained pressure regulators are essentially proportional controllers. There are two primary categories - pressure reducing and pressure relieving (back-pressure). These are ideal for constant loads. Change the conditions and the regulator responds, but with an offset.

A back-pressure or pressure relieving regulator as a proportional only control valve that responds to changes in the inlet pressure. Force from the inlet pressure is exerted on a diaphragm in opposition to a spring, and the inlet force is applied to the top of the disc forcing the disc off the seat thus increasing flow. An opposite force from the outlet line is excerted against the disc toward the diaphragm tending to lift the disc. The diaphragm has greater area than the top or bottom of the plug. The outlet area of the disc is greater than the inlet area of the with a ratio based upon the stem area. Flow through the regulator would decrease as the outlet pressure increases. An outlet pressure greater than the inlet pressure would force the disc against the seat thus preventing back flow.

The pressure-reducing regulator is similar except that the outlet pressure changes affect the valve opening.

John

 

[Ashereng]

Hi John,

Thanks for your posting.

Any thoughts on the interchangeability between a "pressure control loop setup (pressure element, transmitter, PLC, globe valve) versus a self-contained pressure reducer/regulator" ?

 

[jsummerfield]

Being a control systems engineer I tend toward the complex when more simple may work. For industrial process control applications I prefer a pressure transmitter with a PID type controller and a control valve over a self-contained pressure regulator. I also specify lots of regulators for some applications. I recently bought a bunch of tank blanket regulators. The explanation about proportional only was also provided to the client when they inquired regarding a calibration certificate. The regulator setting is accurate at a particular demand.

The regulator balances forces. If the flow deman changes from the flow at the time that the regulator was set, then the controlled pressure will be off somewhat from the setpoint. This is true for proportional only applications whether simply mechanical balance or using pneumatic or electronic control set for proportional and without integral.

You must balance the application regarding the constancy of demand and how sensitive the application is to setpoint offset.

Two-inch is toward the high end of the available selections for self-contained pressure regulators. Is instrument air available for the valve? Is there a PLC or DCS available with PID control capability? Are there severe lenght issues for the signals that might lead your decision?

If selecting the regulator I would begin reviewing the literature from Fisher, Cashco, etc. to select a model - then discuss the application with a peddler.

John

 

[JimCasey]

As John says, 2" is pretty big for a single-stage regulator. A 2" regulator generally does not flow as much as a 3" globe control valve because it will need a metal diaphragm--very short stroke/lower capacity.

regulators have an interesting characteristic known as "inverse sympathetic ratio" also known as supply pressure effect. As you raise the supply pressure,the internal force balance of the regulator changes and the outlet pressure of the regulator decreases by an amount equal to the area of the valve seat divided by the effective area of the diaphragm. Large regulators tend to have larger ISRs, so they are not as precise.

Also as pointed out earlier, regulators are basically a proportional-only control element and any change in the operating point is accompanied in an offset of the output pressure. 10% of span change in pressure between low flow and max flow is reasonably typical for a direct-acting regulator. So

If your "sweet Hydrocarbon gas" is noncondensing and clean, you might be able to use one of the pilot-operated regulators offered by Spence or Leslie, although these are have materials optimized for steam. Pilot-0perated regulators tend to have relatively tiny ISR, and very high gain compared to direct-acting regulators.
The guys at Kaye&Macdonald (Cashco) have a modular line of regulators and they can piece-together a piloted or dome-loaded regulator with pretty impressive response. Of course there is a crossover point where you can spend more dressing up a sow's ear than you would have spent on a silk purse in the first place.

 

[JimCasey]

OK-Correcting typos before somebody does it for me.

Paragraph 1 should compae a 2" regulator to a 2" globe.

Metal diaphragm: used at higher pressures and temperatures. Your example application could use an elastomer diaphragm. Still the stroke tends to be short due to the practical constraints of spring selection.

Paragraph 3: Truncated. last sentence should read :
So if the spring range is 10-150 psi, there could be a 15 psi difference between the zero flow setpoint and the output pressure of the regulator at full flow.

Unfortunately this forum does not have an "edit" feature.

 

[Ashereng]

No worries Jim.

I wish this site had a spell checker too.