Parallel slide valves in HC service?? Your experiences? Recommended?

[robsalv]

I've had a question from a manager, asking what I thought about parallel slide valves in HC isolation service.

I don't think they're a good idea in HC services since they are only leak tight whilst there is upstream pressure. Lose the pressure and their ability to isolate effectively is compromised.

Having said that, it got me thinking whether they might indeed be used in such services. To my knowledge, parallel slides are primarily used in steam and hot condensate services... but I could have a skewed view.

I would appreciate your experiences.

Thanks in advance.

Rob

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[gerhardl]

If you by parallell slide valves mean the types knifegate valves, there exists, mostly used for waste water, valves dropthight in both directions from 0 barg to 10 barg, lower for sizes normally above 300mm (12 inches size).

You also have O-port and knifegate valve constructions for wood, pulp and paper industri, also thight under all conditions, often stainless and at high price-level.

Apart from this I agree with you, I dont see why you should select theese type of valves for HC service, when there exists a vast number of better suited valves for the purpose.

Normally the idea of using an odd type of valve for an 'unnormal' purpose pops up as a suggested solution for problems outside the process itself: saving money in general by low-cost products with short lifetime or unsure performance or try to avoid cost of necessary (re)building of pipelines by 'bending rules' are the most common.

 

[KaiserSauzee]

Perhaps your manager had in mind the double expanding gate valve type. Somewhat similar to the parallel slide however the slab is split in two parts, each with a v-shape wedge like profile. When one part of the gate meets the end-stop the profile forces the "slabs" outwards achieving a seal (i.e. not pressure dependant like the regular slab type parallel slide). Add a bleed on the valve body and you can have double block and bleed (Note: I have never actually used this functionality).

Mostly these valves are used for "dirty" HC services in a subsea environment.

 

[robsalv]

Thanks for your comments gents. I believe we have one of these valves in storage and a need to replace an inservice valve... so I suspect there's a cost motive and also a testing the boundary motive in the manager's question.

The type of valve is as described in the pdf at this link:

http://www.tycoflowcontrol-na.com/ld/DEWDR-0002-EN.pdf

Even the literature spells out the typical applications as:
- Main Steam Isolation
- Boiler Feed Pump Isolation
- H.P. Feed Heater Isolation & By-Pass
- Blow Down Service
- Blow Off Service
- Gland Steam System Drains
- Steam Turbine Inlet Isolation
- Economizer Recirculation
- Spray Water

...kind of reinforces my original thinking.

- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[gerhardl]

Seen your illustration.

This is a variaton of the O-port 'gatevalve' type in a closed housing. The port is closed by the solid upper part of the inner 'slab' positioned between inlet and outlet, and opened when the O-formed opening placed in the lower part of slab when the slab is slided upwards, all parts in the closed housing all the time.

Even without knowing your service conditions I will definitely not recommend this type, I can see a number of possible problems. I think your description of typical applications for this valve says it all.