3 Way v 2 Way valves.. is their a sealing effectiveness difference

[jamesbanda]

All,

I've heard many operators state that 3 way valves dont seal as well as 2 way valves or act as well for isolation. Particually on large duties..


is this a fair statement.. or is it just urban myth based on word of mouth..

 

[moltenmetal]

It seems fairly evident from the differential pressure ratings of equivalent 2-way and 3-way ball valves. But there are problems which are far more readily and safely solved with 3 ways than they can be with combinations of 2 ways in my experience. Tightness of isolation isn't everything.

 

[zdas04]

I've had the same experience as moltenmetal--getting two on-off valves to play perfectly together can be a challenge.

David

 

[gerhardl]

Let me expand a bit on above answers. The question asked can be answered in a number of ways depending on what size, pressure and applications we are talking about.

A brief summary from my experience:

1. Regulating purpose: generally as answered by moltenmetal and Zdas04, especially if readily avilable valves(competition on the market) could be found. This is often the case for 'smaller' sizes, 'lower' pressure classes, and fluids as common steam, warm water mixing etc. (Some good solutions for higher pressure classes/smaller valves higher temperatures, and larger valves exists though)

Theese types give correctly dimensioned, installed and adjustet good regulation and satisfactory sealing, but may over time not shut thight. Should always be combined with on/off valves as revision or shut-off valves. (Some types with excellent sealing properties over time exists)

2. Larger regulating valves, higher pressure classes and pure on/off valves, and 'non standard' applications (one or more of theese factors combined).

Here we have more limited access on the market for valves. Three-way valves of this type could be difficult to replace at short notice and might be higher in price than two single valves, and may also have (especially for older three-way valves) non-standard and differing building lengths.

In theese cases three-way valves should be avoided. The effort should be moved to obtain best type two-way valve (regulating or on/off as suitable), correctly installed, dimensioned and adjustet (correctly removed measuring point for in-signal, damped regulation, and valves not working 'against' each other).

With correct solution the standing time before the valves no longer are dropthight would probably be longer than for three-way valves, but again depending on particulars.