While gate valves and knife gate valves share many similar characteristics, there are some fundamental differences that set them apart. The following we will explores the similarities and differences between these gate valves.
HOW DO GATE VALVES AND KNIFE GATE VALVES WORK?
Gate valves open by lifting a round or rectangular gate (or wedge) out of the path of the fluid. The sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is needed. They have a flat gate closure that slides in and out or up and down between two parallel plates to open or close the valve. Gate valves are used for shut-off, rather than flow regulation because they only have two settings: on and off.
Knife gate valves perform exactly as the name implies. These types of valve 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. Knife gate valves were designed with a sharp edge to cut through the pulp and seal. With this type of useful attribute, the knife gate valve has become invaluable in applications involving slurry, viscous fluids and other systems where impingement is an issue.
GATE VALVE AND KNIFE GATE VALVE APPLICATIONS
Gate valves are used in many industrial applications requiring shut-off valves, including the oil and gas industry, pharmaceuticals, manufacturing and automotive. They can cope in demanding environments, such as high temperature and high pressure environments. Common gate valve applications include in power plants, water treatments, mining and offshore applications.
Knife gate valves are advantageous in sludge and slurry applications because their blades cut right through thick liquids easily. They&#;re generally specified in larger sizes for handling thick flows of heavy oils, light grease, slurry, paper pulp, varnish and wastewater to name but a few knife gate valve applications.
ADVANTAGES AND DISADVANTAGES OF GATE AND KNIFE GATE VALVES
One of the advantages of gate valves is that pressure drop across a gate valve is very low when it is fully open. They can also be used for bi-directional action and are useful as on-off valves. Gate valves do, however, need a large force to operate and large sized valves need automatic actuators. They&#;re not quick to open or close, and take up more space, when compared to some other valves. Thermal expansion and shrinking can also lead to unwanted leakage in some gate valves that are exposed to high temperature fluctuations.
The advantages of knife gate valves are that they&#;re cheap, easy to actuate, and light. One of the most notable disadvantages of knife gate valves is that they&#;re known to have low-pressure limitations. This makes them less desirable for use in applications that require cleanliness or sanitary conditions.
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View Full Version : Valve questions
Ellie
I'm ready to start work on the SC hook up. I didn't find and 3" valves in my DH's yard so I'll have to buy one but there are so many choices: ball valves, gate valves, knife valves, check valves, etc. and then there's the with unions, without unions...
Can someone please tell me what I need?
I have 3" pvc from BD that will enter a 55 gal drum SC.
icu2
I'm far from an expert, but if you're not trying to control the amount of flow, I'd think a gate (also called a knife... same thing) valve would do the trick. They are "all on" or "all off" type. I've never seen any come with unions (which are screw on, screw off type of connection... so you can easily disassemble things) but Fernco's are rubber fittings with hose clamps and they can be used for a fraction of the cost of buying unions... Check valves allow water to flow only one way... just an internal thing... no handle or anything. Ball valves will allow you to control the amount of flow... like a faucet in the kitchen.
Hope that helps!
Steve
Ellie
Ahhh. I see now. That makes it so much easier to understand. Thank you.
boggen
there is a difference between gate valve and knife valve.
gate valves. have a twisting knob. that causes the peice inside the valve to go up and down. in idea the handle is like a big bolt. that screws in and out. normally it takes multipule turns of handle to full open / close the valve.
ball valve = a twisting noob. that normally only takes 1/4 of a turn to close or open the valve all the way.
knife valves, have a handle that moves straight up and down. that causes the peice inside to move.
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you may have problems finding any type of 3" valve locally. or less ya dig out the book and start calling around and asking.
it may be easier just to order the 3" valve online.
icu2
Thanks Ryan!
I learned something new today! :clap: I'd always heard terms knife and gate valves used interchangeably... are the gate valves designed to be used to limit flow, or should they just be used to totally open, or totally close the line? Or do they care?
And just as a side note for Ellie, I don't think the gate valves often come in the plastic (pvc) variety... the ones I've seen are mostly the metal kind. The knife ones are the more common ones you'll find used... kind of work like a guillotine! :eek1:
Steve
birdman
Yaxing Valve contains other products and information you need, so please check it out.
My local RV store sells "Valterra" knife valves in 1 1/2, 2, and 3 inch sizes, and they can special order 4". These are pretty darn nice valves. The 3 inch cost me $18. Not bad. Here's a whole herd of 2 and 3 inchers, and a close up of a 3 incher.
Harveythekoi
I don't see any green transition glue on those. The black Valterra ones are ABS not PVC. Not that they work any different but you should use the transition glue when going from one plastic to the other.
Garrett
icu2
These are pretty darn nice valves.
That makes me feel better... I just picked up 3 of them for $20 inc. shipping! :yahoo: But wasn't sure if they were any good. But I figured for that price, I'd risk it.
And good info Garrett... I'll make a mental :rolleyes: note!
Steve
Ellie
Is the green transition glue the same as the "ALL PURPOSE" pipe cement. The can says its for PVC, ABS, and something else? I saw that at Lowes today when I bought the PVC cement and primer.
Harveythekoi
No, will it work, Yes. That universal glue is not a solvent weld process like most of the others. Don't really care for it myself but I'm sure it would be fine.
Out here to meet code if you switch between PVC & ABS for sewer use the green transition glue has to be used. Probably why they colored it green so an inspector can see with a glance that the right stuff was used.
Garrett
AlRon2
I don't think ball valves are made to be used to control flow. I though they were used like knife valves (either off or full on). Can somone help out here?
Thanks
Ron
Harveythekoi
I don't think ball valves are made to be used to control flow. I though they were used like knife valves (either off or full on). Can somone help out here?
Thanks
Ron
Made for it no.....work in that function yes. The common thinking is the cheaper knife valves with the plastic paddles could distort and not seal shut if used in a partially closed position. The stainless ones will usually work fine like that but have been known to drift closed some.
Ball valves aren't the best either but seem to stay in place and by design will not warp inside.
The best to control flow are butterfly valves but no one makes them cheaply in smaller sizes and do a poor job of closing entirely. They are the least disruptive to flow when blocking the path.
For our flow rates etc and cost ball valves usually fit the bill. Most flow is controlled right after the pump in 2" size pipe so the valves are cheap enough.
Garrett
AlRon2
Thanks Garrett.
Ron
Broc
You haven't specified how you're going to use the valve re: your SC.
If you want to be able to control the flow through the BD to the SC, you will probably want a ball valve. The knife valve is an all-or-nothing... The ball valve is adjustable.
But -- and this is important for those who may not know -- don't make the mistake of NOT putting any kind of valve between the BD and the Settle... You WILL want to stop all flow into the SC at times... like... uh... the "ever-so-offen" clean out...
~ B
:yes:
birdman
I don't see any green transition glue on those. The black Valterra ones are ABS not PVC. Not that they work any different but you should use the transition glue when going from one plastic to the other.
Garrett
Boy thats a new one for me Garrett. I use the clear all purpose glue. I have never seen green transistion glue. I'll have to go look for some, maybe try some different plumbing suppliers. Thanks for the tip.
Pond James_Pond
What about Red Hot Blue Glue? I thought that went both ways?
steve
Harveythekoi
What about Red Hot Blue Glue? I thought that went both ways?
steve
It's listed as a PVC glue, no mention of any other plastics. Not to say it won't hold but the chemicals that melt PVC for the solvent weld process will not effect ABS the same way. While it feels like a solid joint I doubt it's actually welded. But it is a thick glue and that alone could keep things in place for our pressures.
Garrett
wfhsdemons
Are knife valves ok onthe BD Line and Skimmer line? Is there ever a time you need to control the flow at that point?
icu2
Are knife valves ok onthe BD Line and Skimmer line? Is there ever a time you need to control the flow at that point?
like if you've got 2 BD's that are very different in length (i.e., head), then you'd have the ability to adjust the flow to even them out...
But if everything is pretty close, I'd go with knife valves. Cheaper and much easier to open and close!! :cool3:
Steve
If you are looking for more details, kindly visit knife valve vs gate valve.