Pressure reducing valve AND flow control valve in series for steam to water HX?

[MinnesotaSlinger]

Hello. I've got a project where I'm going to take steam from 80-100 psig (the header pressure varies somewhat) and deliver it to a shell-and-tube HX. Earlier in the design process, I assumed that I would put in a PRV to reduce pressure to 50 psig, and with the HX I have quoted for the project, it is sized assuming 50 psig. (The design pressure for the unit is 150 psig.) At 50 psig and the values I provided for required heat transfer, water inlet temp, and water flow rate, I have a unit quoted that meets those conditions.

Between the PRV and the HX, I've assumed that I would install a control valve that is controlled by outlet water temperature. By having a PRV and control valve is series, I will have knocked steam pressure down to my design condition of 50 psig and will be able to regulate flow into the HX to maintain my necessary HX. But something about having a PRV and a control valve in series strikes me as sort of dumb and a waste of money. Would I really need a PRV ahead of the control valve? I don't really know that there is any harm in pressure going above my 50 psig design parameter. And assuming that the HX is operating close to design conditions, I would think the one valve would hold back flow such that the pressure in the HX would be about 50 psig, a little more or less depending on, say, how clean the tubes are.

Anyway, if you have a steam header that varies in pressure, should you just get a HX sized and capable of operating at the low end of that range? (I could go back to the vendor and size for 80 psig.) Should you get it sized for a lower pressure and put in a PRV to drop pressure to that level ahead of a separate control valve (and in so doing, you wouldn't be subject to pressure fluctuations in the steam header)? Can you just size a HX based on some pressure lower than the low end of the steam header pressure, skip installing a PRV, and just let a control valve control steam flow?

Words of wisdom about how to ensure proper operation of an F&T steam trap and other condensate concerns related to strict or unrestricted HX pressure control is also appreciated. Thanks.

 

[BigInch]

It doesn't make sense to me, unless you've got 100 miles of pipe between them, or other reason that they won't fight each other.

The prv will essentially vary flow as it tries to keep the downstream pressure below 50. Closing as the pressure tries to rise, or opening when the pressure decreases. Your flow controller valve will work with the prv, as long as it doesn't try to send too much flow to the prv, in which case the prv will override whatever the flow controller says as it always tries to hold the 50 downstream. It will not let any of the excess flow sent by the flow control valve through it, which would essentially limit the same flowrate to go through the flow control valve.

The flow control valve would be able to work within a flow range of 0 up to the constraining flow equal to whatever the maximum flow that results from the prv trying to hold the 50 psig downstream.

In other words, why not
1.) eliminate the flow controller, or
2.) turn the flow controller into a downstream pressure controller and eliminate the prv.

There is no difference between a flow control valve, or a pressure control valve, or a prv, except what signal is controlling it. It is usually either pressure, or flow, but sometimes it's temperature, or other variable.. %Butane into a Methane stream, or even color? Want to control the mixture making red paint? Whatever. Basically a valve is a valve is a valve.

I have used two pressure control valves in close series, but only because one valve would not do the maximum total pressure cut of 2160 psi without eating itself to pieces in 90 days.


Independent events are seldomly independent.

 

[Compositepro]

In your process a common problem is that when the water temperature is too cool the valve will open and steam pressure in the heat exchanger will rise to header pressure before the water reaches your set temperature. Then the water temp. overshoots set point and the control valve shuts-off the steam. Steam pressure is much easier to control accurately since feedback is instantaneous. Cascade control will use water temperature to adjust the steam pressure set point. Absent doing this using a pressure regulator can help to reduce the temperature over-shoot. Which way to go depends on how accurate you want the temperature, how much the water flow rate varies, and economics.

 

[MortenA]

a valve is a vale is a vale! The valve dosnt "know" that its a pressure reducing or a flow controlling valve. Its your control system that sets that! So you cant have two valves in a series like that and expect good control (as biginch also states).

 

[LittleInch]

There's a few things here which don't make sense to me at the moment.

If your steam pressure is 80 -100 psig and you've decided to aim for 50 psig, why is your vessel designed for 150?? DO you like throwing money away or is there a good reason for it??

If you want to have a very stable steam pressure then fiar enough go for 50 or 60 or 70 psig and control steam flow for both presusre and flow with a singel control valve with two inouts inot the positioner control loop. Very common and saves on a valve and any "fighting" between the two. If you do put two control valve sin series, you need to mkae one of them much slower in response than the other to avoid hunting as they react to each other.

How welel your system responds will be governed by how much and how fast your heat load changes. If it's relatively slow then you should get good accuracy and control. If it fluctuates wildly and quickly, then it will be more difficult to get accurate wate rout temperature.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DRWeig]

Hospitals everywhere: 150 psig steam to a shell-n-tube heat exchanger to make hot water for either space heating, domestic use, or both.

Pipe 2 valves in parallel (not series!), one sized for about 1/3 of design steam flow and the second sized for about 2/3 design steam flow. At low loads, the small valve gives good control. At higher loads, the small valve stays wide open and the larger valve provides for good control.

Is this a good alternative to your two-stage serial control?

Best to you,

Goober Dave

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[DRWeig]

Forgot to mention - if your heat exchanger is sized for 50 psig steam, no worries at all. Size your valves for the higher pressure, and the control loop will take care of it.

These valve pairs are often controlled by a single PID loop -- the small valve operates at the lower 1/3 of the control range, and the large valve operates at the upper end. We did this with pneumatic actuators and springs with a single controller in the olden days. With DDC, it's a matter of programming two independent loops.

However, it is a bit of a waste to have a 150 psig exchanger sized for 50 psig as LittleInch said.

Best to you,

Goober Dave

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[MinnesotaSlinger]

Thanks everyone for the great advice. It seems to make a lot more sense to just have one valve to control flow. We will have both water outlet temperature and HX steam pressure inputted into the plant's Ovation system, so we should be able to control off of water outlet temperature while using steam pressure to prevent overshoot during S/U and other transient conditions.