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.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.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).

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