We have a water booster pump circuit which has suction, discharge and bypass valves (all 24"). The water is mixed upstream of this pump with caustic due to process requirement (pH = 8-8.5, Sp. Gravity = 0.97, Pressure at suction is 60 psi and discharge is 100 psi, Temp. is around 90 C and flow through these valves is around 1700 l/s)> The pump is installed inside a building but all the valves are installed outside and ambient temperature range for the valves is 40 to -50 C. Water also carries 0.5% suspended clay/sand in it.
We currently use full body pneumatic Dezurik Knife gate valves with metal seat and S.S. gate with carbon steel body. The packing is Braided PTFE.
All three valves have two chronic problems:
1) During the actuation valves cease.
2) The packing leaks.
One reason of valve ceasing is that during the operation scaling is formed on the gate and is hardened.
Packing leakage is not such a big deal during summer time but during winter it forms ice around the valve and cause valve body to crack. Plus it also steam clouds which cause poor visibility and safety concerns.

Is there any solution for my problems? We have tried several things in last 4-5 years with or without valve manufacturer to no avail. Is there any replacement that you people can suggest? Is knife gate is the better option than wedge gate or butterfly vavle (it's on-off situation only). Can we try an oversized actuator to overcome scaling and stiction problem? is PTFE is the suitable for -50 C ambient and 90 C process? I appreciate your help in advance.

If scaling is causing a knife gate to seize, I would think that a butterfly (disc) valve will be subjectd to the same problem - scaling on the sealing surfaces causing the valve to jam (not close completely).

Have you tried partial stroking? That may work to dislodge some of the scaling?

Or, can you inject a solvent once in a while to dissolve the scale? You may need a parallel valve station (I know, not cheap at 24") to do this.

Orbit ball valves have a feature that lifts the ball off the seat befoe the 90 degree action takes place. They are used where incrustaceans take place.

An eccentric plug valve would also suit. Dezzurik make these too.

Knife gate valves do leak. They are designed for sewage and slurries where the odd leak is tolerated. It would appear someone has selected the valve on pricenot performance and you are the Bunny stuck with the problem.

I would look at a lined Butterfly valve for this application.  For water use an EPDM liner, disc could be cast iron.  There is no packing in a lined butterfly-the lining is an interference fit with the shaft where it passes through the body.  Install it with the shaft horizontal and with the actuator on the left-hand side when you are looking downsteam.  The bottom of the vane will then open toward the upstream direction and push the layer of sediment away from the seat when the valve opens. Also, the bottom bearing will not tend to accumulate grit.   The liner flexes when the valve cycles and will crack off the deposits that form.  Some vendors to consult are Tyco/Keystone, Bray, Demco, Crane, but there are many others.   

As pointed out earlier, knife-gate valves are really only suitable for heavy slurries such as paper stock and sewage because knifegate valves slice off the stringy components in the fluid when they close.  Dragging a wide, contaminated blade 24 inches through a big wad of packing is just asking for leaks.

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