Knife Gate Valves
Knife Gate Valves
(OP)
Hello Valve experts!
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 (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.
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 (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.





RE: Knife Gate Valves
To prevent ice forming, and causing packing leak, why aren't you heat tracing and insulating? I would think that would be a simple solution.
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.
Interesting problem.
RE: Knife Gate Valves
If you want to keep you current valves, there might be a few options. What is the water for....can you add a descaling caustic? It wouldn't take much. If you are re-injecting the water, this is a good idea. as for the packing, I would think that teflon is not the best option. Not sure what to use here...perhaps buna, or a buna/teflon alteration. I would have to contact somebody like sparton and ask a few questions for that.
RE: Knife Gate Valves
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.
RE: Knife Gate Valves
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.