Gate valves to control or regulate flow

[Alnoel]

Hi. I am currently working on a geothermal project that will drill new Wells to be hooked up to the existing SGS (steam field gathering system). I have a query regarding the utilisation of gate valves as flow control though I understand that these type of valves are not designed to control or regulate flow. Reason i am asking this is, we initially considered a triple offset butterfly valve as a means to control flow , apparently large pressure drops are expected, at this type of valve, even at full open condition. For geothermal operation, large pressure drops results to lower productivity/deliverability from a producer well. One of the reasons why we need to control flow at the well is if potentialy we encounter a surging flow type of well (which we have already experienced) which could result failures on our cross country piping, and issues on our brine production management at our downstream facility.
My question is, if we are to consider a gate valve as means to control flow (this is downstream of slab type gate valve) which we consider as a sacrificial valve, would a wedge type be better as compared to a slab type one (considering cost, ease in operation and longevity)?

 

[Danlap]

Hello,
IMO you and team should assess the pros and cons of using gate valve as “throttling function”. First understanding why you should not use gate valve, and use that screening methods to tune your expectation and mitigation:
-Opening gate more than 60-70 percent usually considered as full flow, as it is nearly impossible to exactly control the flow between this value and fully open. So use gate valve for opening less than 60%??? There are some factors to be considered as well
-Gate valve with small opening might induced vortex shading behind the wedge thus your spindle might vibrate and gland packing leakage.
-Gate valve with small opening might increase the velocity of product. Magic numbers of 8 m/s considered might start to erode the sealing element of the wedge. Any type (double expanding, wedge, or slab) will still suffer from this.Thus if you already have eroded wedge, then your gate valve in ‘close’ position for sure will have internal leakage. Some application might requires less velocity to erode metallic components.

Your question “cost, ease in operation and longevity” are often contradict one another. If longevity meaning capability of fully close without any leakage? Then it is hard to justified. Which type will survive longer and seal better, requires trial and error and proper hard facing selection.
Ease in operation? Slab type have higher friction, thus might requires bigger (mechanical advantage) gearbox or actuator
Cost? Wedge type is the cheapest of all
For sacrificial method, usually Plants use wedge gate valve which often readily available.
For non-abrasive application, small numbers of manufacturer dare to design you a control gate valve.

regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[LittleInch]

If you're going that way, why not use a V notch ball valve or similar?

Any type of gate valve will perform equally badly as a control valve and last about as long.

I'm more concerned that you have a known situation which as you put it " (which we have already experienced) which could result failures on our cross country piping,". You need to address this hazard first and do it properly, not simply putting in a sticking plaster when dealing with (high pressure?) steam in a cross country situation. IMHO.

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

 

[Alnoel]

Hi Littleinch, thanks for the response. Yes, the ball valve was one of the consideration back then when we had that issue on one of our surging well. Apparently the cost of procuring such valve (I. E. 16inch class 900) superseded the benefit plus the longer lead time from our accredited suppliers further worsen the economics involve. Thus the suggestion of putting a sacrificial valve. If I am not mistaken, we just achieved an ideal opening at the sacrificial valve so as to prevent the surging flow from the well, it is then left at that percent opening condition. Since this is considered as a sacrificial valve, this component will not be considered as a main isolation point hence the existing adjacent slab gate valve is installed to perform that feature. This is just a contingency, in case we don't achieve any surging condition at our newly drilled Wells, this valve will be left (fixed, 100%) open during normal operating conditions.

 

[Alnoel]

Hi Danlap, thanks as well for the response and input. Our contractor do suggest a wedge type gv will be suitable for this application and the intended sacrificial condition. Referencing my response to littleinch above, we did perform that assessment and we initially recommended a tov long flanged butterfly valve or a ball valve (lower dp compared to bv). But due to associated cost and lead time, we ended up using a sacrificial gv. We did try to fix the problem by increasing or further reinforcing the downstream pipe supports still is not sufficient to address the dynamic loads brought by this surging well.

 

[RVAmeche]

So this valve will typically be 100% open and if you have a surge event, the intent is to partially close the valve to prevent the surge from reaching downstream?

I would say instead of investigating this "sacrificial valve" as a half measure, you should instead implement changes to actively prevent another surge event. Maybe surge suppressors, modified actuated valve timing, etc depending on the cause of the surge.

 

[KoachCSR]

I would caution you against the use of a gate valve for this scenario.

One likely failure scenario for a gate valve subjected to severe duty scenarios such as the one you are describing (mitigating a surge) is that the wedge separates from the stem - the wedge will then fall into the seat, effectively causing the valve to permanently shut with no way to recover short of disassembly. The stem to wedge connection of a gate valve is typically a weak point which will see a lot of stress during throttling.

What kind of pressure drops are you expecting with a butterfly valve? For the line size you mention, I would expect that a butterfly valve would be the appropriate type.

 

[Alnoel]

Hi RvAmeche,

Thanks for the response. Yes, valve opening will only be adjusted if we encounter this on the affected well.

Hi KoachCSR,
Thanks for the response. Would a flexible type somewhat mitigate this from happening? One of the precautionary measures we have considered is to prevent the valve from full shut-in. In case of downtime and/or SD conditions requiring isolation, valve will be fully opened and the upstream isolation gate valve will be the one to be isolated from the source. For the pressure drop, the concern is more on the resulting reduced production brought by high pressure drop from the source.

Thank you all for the valuable inputs, I have requested my team now to re-evaluate the consideration of gate valves as a means to control flow; given the scenarios you guys highlighted. I am re-considering putting into the table again the Butterfly valves and Ball valves as an alternative to this. Thanks again guys!

Much Respect, Al

 

[KoachCSR]

Flexible wedge gate valves are designed to provide more positive sealing at both low and high pressure differentials - with isolation service in mind. I'm not aware of a gate valve design with a strengthened stem-to-wedge connection. I believe you'll find that most manufacturers warn against using gate valves for throttling service.

I believe you're making the right decision by having your team re-evaluate using a gate valve - I would recommend your team inquire with some valve manufacturers and see what they can offer with respect to high performance butterfly valves. Fisher, Powell, and Crane come to mind as some manufacturers you may want to reach out to initially (though there are several others too).

Good luck, Al.