Difference betwn knife & slide Gate valves 1

[pmover]

all,

what are the diffences between & applications of gate valves and slide gate valves? i know they are used for slurry and abrasive fluid systems, but what are the differences between them.

i am searching for an answer myself, but i believe there are others whom may be interested as well.

thanks!
-pmover

 

[pmover]

also, what are the advantages/disadvantages for both valves?

thanks!
-pmover

 

[MikeONeill]

A knife gate valve is just what it implies. Knife gate valves were originally designed for the pulp and paper industry. Stringy pulp would impinge between the wedge and seat of a normal gate valve and prevent flow shut-off. The knife gate valve was designed with a sharp edge to cut through the pulp and seal. Due to this, the knife gate has found a home in slurry, ash, and other systems where impingement is a problem.

'Standard' gate valves use a solid, or flexible wedge design to seal. the wedge is forced into a V-shaped seat and sealing is accomplished by virtue of the brute force utilized to drive the wedge into the seats. Thermal binding is an issue with wedge gates.

Parallel slide gate valves utilize 2 parallel seats as opposed to the V-shape of wedge gates. This design bases sealing not on torque, but on the spreading action of the 2 disc faces as the valve seats bottom out. It also utilizes the motive pressure of the media to seal the downstrean disc face. In theory, the higher the pressure, the better the seal. With slide gates, only one of the two seats are actually sealed. This type valve is being used more often now for high temperature steam systems as thermal binding is not a problem.

In terms of advantages/disadvantages, knife gate valves are generally limited to ANSI class 150. They are cheap, easy to actuate, and light. sealing is by either metal to metal, or an elastomer o-ring seal. They are unidirectional shut off valves. Wedge gate valves are available to ANSI class 4500 and provive reasonable shut-off in both directions.

Hope this helps a little.

 

[pmover]

Mike,

Thanks for the posting - useful!, particularly the info on knife gate valves.

i did not proof read my question until now - shame on me.

the comparison i wanted is for knife gate and slide gate valves. i now understand the contents of your posting.

for the curious individuals, the application is for coal ash hoppers, valves installed in vertical postion (meaning vertical piping system).

further explanation for uses and advantages/disadvantages is requested.

apologys for not proof-reading questions b4 posting.

-pmover

 

[MikeONeill]

pmover,

For coal ash hoppers I'd recommend a metal-seated knife gate valve with grafoil packing. They do a good job slicing through, and sealing, againt ash. They're used with much success in the New England area. Yes, they'll wear out eventually, but the pricing and ease of installation offsets that. Besides, even the best fully stellited slide gate wouldn't any longer. One of the options you can request on a knife gate valve is stellited seats. It's a chunky price difference, and to be honest I've not seen more than a 20% field life extension in using stellite. Let me know if you need more info.

Regards,
Mike

 

[NGiLuzzu]

Just to get an idea of knife gate valves, take a look at

http://www.knifegate.com

Hope this helps, 'NGL