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Gate valve: solid or flexible wedgeHelpful Member!(5) 

UliR (Mechanical) (OP)
27 Feb 07 2:19
Dear all,
what are the reasons to use solid or flexible wedges in chemical service for gate valves.
I only found that at small sizes I have to use solid (<NPS 4) NPS 6 and large shall have a one pice flexible wedge. Why??
 Thanks

UliR
Helpful Member!(2)  gerhardl (Mechanical)
27 Feb 07 3:02
Your choice of valves have always to be taken considering the limitations and availabilities given. Most important:

- Process, function, material, fluid and physical requirements and details
- Commercial availability
- Official and factory safety limitations and requirements
 
Within this framework your choice should end up with the valve with the best or optimum cost over lifetime.

Starting with commercial availability and checking against your other parameters, you could well end up with the answer to your question that the reason is production cost for valves and commercial competition among valve producers, and no real technical reason related to your process.

Searching the world-wide market you could even end up with the same conclusion the other way: both types available and possible to use, but the one more competitive or more commonly used for your process or in your area, or for this specific pressure class and connection type.

As for all valve questions: full process and sizing details needed for specific comments.

Helpful Member!  UliR (Mechanical) (OP)
27 Feb 07 3:24
Thanks gerhardl,

sorry for beeing less detailed.
More detailed question:
Why exists such wedges? Which processes or fluids use solid which use flexible wedges? Is this due to closeness, density, viscosity, temperature, corrosion, abrasion...?

best regards
UliR
gerhardl (Mechanical)
27 Feb 07 5:12
Definition: Flexible wedge - > A wedge normally coated totally with special grade elastomer, usually rubber types NBR or EPDM based. Solid wedge: metal wedge with metal to metal seating. There are some hundreds (thousends?) of types and variations, but on a general basis (European):

Soft sealing gate valves (flexible wedge gate valves) main group from DN40(mm) up to most commonly used 250mm to 300mm (6-8inches) but readily avilable up to at least 1200mm. Pressure classes: low (PN 10 and PN16 (bars), mostly PN6 or PN10 above 8 inches but commercially available up to PN16 (All these classes below ANSI 150)

This group is most commonly used for drinking water and sewage water, and will seal thight with relatively long lifetime and low maintenance requirements, and have a low price, often lowest for any valve in range up to DN250. They are also allowed and used for natural gas (household gas) distribution, air conditioning, pressurized air, and other types of water distribution and neutral fluids. Usually housing (used as seating) is inside epoxy coated or (more limited) emailled.

Below these sizes rubber coated wedges are not practical or economical to produce.

For higher pressure classes or other media the rubber coating will/could give problems (gas bubble expansion within coating, not be compatible to temperature or fluids etc), and are of this reason not easily commercial available or selected produced.

The large group of metal seated gate valves have the minus that the sealing surfaces are more exposed to encrustations, corrosion, abrasion or cavitation (by not properly closed valves)than soft sealing gate valves.

If a soft sealing and a metal to metal sealing gate valve both hit the requirements of your application the soft sealing would normally be the cheapest, more mass-produced item. Be however aware of flow speed limitations, which is commonly lower for the soft sealing gate valve.

Also: depending on your chemical and total application requirements, other type of valves than gate valves could be considered, again also depending mainly on pressure class and size.

UliR (Mechanical) (OP)
27 Feb 07 7:41
Thanks gerhardl

this was a helpful answer.
An additional answer for my question I received from another source.
A flexible wedge used for our services is a non-coated metal type seated on a ball. This has the advantage of guaranteed sealing of both sides of the seat. It is used in petrochemical service. The space inbetween the wedge can be leak-detected to ensure the closeness of the valve.

best regards
UliR


Helpful Member!(2)  JimCasey (Mechanical)
27 Feb 07 12:31
I disagree with both definitions for the flexible wedge.  

IN A GATE VALVE a flexible wedge has a cut around its perimeter.  The function is to allow the seating surfaces of the disc to flex a bit so they can conform to the seats and shut off tightly, while allowing for a bit of deflection from piping loads, thermal distortion, machining tolerance.  A solid wedge just has to be brute-forced into the seats and may not shut off as tight.  

Gerhard's coated wedge could be useful for low-pressure, low temperature applications, but rubber coated plug valves are rare. UliR mentioned what sounds to be Top-loaded ball valves such as the McCanna. These  have the seat rings sliding on a taper(wedge) so that they compensate for thermal diatortion or wear.  They are also somewhat process energized, so they seal more tightly with increasing pressure.  
UliR (Mechanical) (OP)
28 Feb 07 8:06
Hi Jim,
you are right, I found a catalogue from Valvosider with pictures from wedges. They differ 4 types of wedges:
- solid, one piece
- flexible, one piece with a cut
- split, two pieces riding on a ball in the middle
- parallel, connected with a spring
Now I have the explanation I was looking for.
thx

jamesb1965 (Mechanical)
2 Mar 07 19:52
UliR
Jim is spot on,
Just for the record a parallel faced gate valve can also be "cam"ed onto the body seat as well as pressure energised, generally they have lower operating thrusts as they are not required to ram into the body wedge.

Parallel gate valves (Parallel slide valves are favoured for steam applications). Some large bore (DN600 plus) parallel faced gated valves are favoured by some markets dam guard / isolation valves as they will not jam like wedge gate valves.

For industrial applications I favour solid wedge gate valves, cheap flexible wedge gate valves can be rammed so hard into the seat wedges that they deform too much and start to leak. I hope this makes sense!

JimCasey (Mechanical)
5 Mar 07 9:11
Going all the way back to the original question:
You mention that this is a CHEMICAL APPLICATION.
Gate valves are cheap when you buy them by the case and they are made from brass or Iron. Good for Water-Oil-Gas (WOG) applications or since there are no elastomers sometimes they are the easy choice for steam.  "Chemical Application" would imply that at least stainless steel would be required.

Ball valves would likely be a more effective choice.  With elastomer seats they shut off tightly. They are easy and quick to actuate. Available in stainless or higher alloys for chemicals. AND THE ROTARY STEM SEAL HAS INHERENTLY MUCH LESS LEAKAGE than the packing of a gate valve.
JAlton (Mechanical)
9 Mar 07 9:43
I agree with Jim Casey on both his explanation of Solid versus Flexible Wedges and his comment regarding Ball Valves Stem Sealing versus Gate Valve Packing.  Our company repairs both Gate Valves and Ball Valves.  Typically, our Chemical Customers use more Ball Valves than Gate Valves.

JAC

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