{Stus-List} Valves - a primer

Bill Bina billbina at sbcglobal.net
Mon Jun 8 10:59:48 EDT 2009


One more issue with gate valves that I've seen: Gate valves are designed 
to be kept either all the way open or all the way closed. They should 
never be used as a "throttle". The reason is that if left partially 
closed, passing liquid and contaminants will erode the gate. Initially 
this will cause it to leak when closed, and if allowed to progress, you 
will eventually have no gate at all. The handle will turn, but there 
will be noting internally to open or close the passage.

Bill Bina


Dennis C. wrote:
>
> FWIW, just a brief primer on valves.  Probably more than most want to 
> know.  See the bottom for commentary on usage of valves in boats.
>
>  
>
> 1/4 turn valves - lever type handles; generally open when handle is 
> aligned with flow; closed when handle is perpendicular to flow
>
>     * Plug valve - has a cylindrical or tapered plug with a hole which
>       is either aligned with flow (open) or across flow (closed)
>     * Ball valve - has a ball with a hole which is either aligned with
>       flow (open) or across flow (closed)
>     * Butterfly valve - has a "wafer" which is either aligned with
>       flow (open) or across flow (closed)
>
> Gate valves
>
>     * True gate - has a round handle.  Rotating the handle causes the
>       gate to move up or down a threaded shaft into (closed) or out of
>       the flow (open)
>     * Swing gate - has a lever which swings the gate across the flow
>       to open or close. 
>
> Knife valve
>
>     * Similar to a gate valve but the "gate" is very thin.  These are
>       commonly seen as sluice gates in drainage applications. 
>       Rotating round handle, threaded shaft
>
> Globe valve
>
>     * Globe valve - has a rotating handle and threaded shaft like a
>       gate valve but instead of a gate it uses a disc which either
>       sits hard on a seat (closed) or rises off the seat to allow flow
>       (open).  Many older bathroom fixtures are a form of globe
>       valve.  I suspect most residential garden hose spigots are globe
>       valves.  If it has a round gasket and a seat, it's probably a
>       globe type valve.
>
> Pinch valve
>
>     * Pinch valve - Rotating handle which squeezes a flexible tube to
>       stop flow. 
>
> Needle valve
>
>     * Rotating handle inserts or extracts a thinly tapered plug along
>       a tapered seat.  Think carburetor idle adjust valve (I know,
>       what's a carburetor?)
>
> Ball valves (and possibly plug valves) are, IMHO, the best choice for 
> boats and are standard for many OEM applications today.  Assuming the 
> handle stem doesn't break, they will always close.  If there is debris 
> or marine growth in the valve, the ball will either slice through it 
> and close or not close at all.  It's obvious whether they're open or 
> closed.
>
>  
>
> Gate and globe valves can present boatowners with a couple of issues.  
> First, because they are a "seating" type valve, debris or marine 
> growth can foul the valve seat.  When this happens, the gate may crush 
> the debris onto the seat and the valve may fail to completely close.  
> Without disassembling the valve, you usually cannot resolve the 
> issue.  Second, if the stem corrodes and shears off or the threads 
> corrode, the handle can spin freely and the gate not move.  The 
> boatowner will not know if the valve is open or shut.  I had this 
> problem on a previous boat.
>
>  
>
> Touche' had several swing gate valves when I bought the boat.  One of 
> them got debris in it and would not fully close.  I have replaced them 
> with ball valves.
>
>  
>
> Dennis C.
>
> Touche' 35-1 #83
>
> Mandeville, LA
>
>  
>
>
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