Knife Valves

[Al601]

What is your experience with knife valves in municipal water pumping stations? Our safety committee wont let us use butterfly valves as a safe block of the energy. We would need to use 2 valves with a bleed between them. we are allowd to use a single gate valve that is locked out but 24" gate valves take up a lot of room, so we are thinking of using knife valves. Any experiences out there?

 

[gerhardl]

Al601,
This is common practise in Scandinavia and Northern Europe, and I have described this solution, and seen it used for a large number of projects, without problems. Installations range from DN50 to DN600 (24") as the normal ready available top size, but more seldom above this size.

Installations have been working for many years with handwheel and spindle only, manual gears, automated pneumatically, oil hydraulic (more seldom) and electrical, and very seldom water hydraulic from drinking water mains. Some with rather long stem extensions.

Range: all types of wastewater installations, pumping stations, pipelines, plants, floodgates, airports with wastewater mixed with de-icing fluid, softer types of concentrated waste with lower water content etc.

There is however a number of important points to be taken care of, briefly stated as selecting products on quality, not on price.

(Search the forum, some of this has been discussed before.)

Most important points listed:

Dropthight both ways from zero to full working pressure (NOT one-sided), no pockets enabling waste or dirt to be packed, valve valid for described working pressure.

Note: flangeforms might be for instance PN10 or PN6 and valve must be qualified for end of line use. Allowed working pressure might be lower (2,5 or 4 bar varying down from PN10 for lower sizes and from brand to brand). Make sure this does not create problems with test pressure, whaterhammer max pressure or other requirements).

Materials: Housing: not less than nodular cast iron, certified epoxy coating not less than 250 my. Gate: not less than SS (good general resistance quality, but not necessarily SS316), stem likewise, good guiding of gate, top quality sealing construction and material and consruction backing for port sealing (if possible steel reinforced rubber).

If automated there is an additional number of points.

Likewise important: correct installation (top up, exceptions as allowed by manufacturor) and regular maintenence: slight greasing on stem and gate.

Supplier should be able to give references from longterm installations.

 

[stanier]

Knife gate valves are routinely used in slurry & sewage applications so there is little reason they could not be used for water. There may be some concern re stagnant water in the chest around the blade. Check out the design to ensure that this is not a problem.

Specify shut off in both directions. Wey or similar valves meet the criteria.

The design has tapped holes in the chest of the valve. If incorrect length screws are used they can lock onto the blade.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[gerhardl]

Sorry, I did overlook the item 'municipal water', however my remarks are the same with some additional points:

1. The valves have been used without problems, but not as far as extensivly as for waste water.

2. Main reasons against: discussions, even without pockets about possible contamination because blade retracts and will be partly exposed to surroundings, and you do have the room for the upper part of the blade where bacteria could grow. Some companies /municipalities and authorities will not allow this type of valve for this reason for purified drinking water on lines downstream for instance UV treatment. OK for untreated (raw) water.

3. Regular maintenance necessary.

4. See my first posting on operating pressure. Might require specially produced knifegate valves with more robust blade to give high enough operating pressure. Accessibel but pricing will be same level or higher than standard gatevalves.

5.If full port is not required cheapest and best valve will be double eccentric flanged BFL valves with short building length (same building length as short gatevalves).

 

[JimCasey]

I have concerns about your "safety" committee prohibiting butterfly valves when the AWWA approves of butterfly valves in municipal water applications almost universally.

Knife-gate valves, as pointed out earlier, work well in what I will politely call "slurry" application because they slice through clumpy or fibrous entrained solids. What they do not do is seal well from the waterway to the environment because the packing box is a long rectangle that presents a lot of leak possibility and the packing is difficult to load uniformly. Fortunately a lot of slurries are self-sealing around small leaks. Gerhardl made a good point when he pointed out the exposed blade surface could drag contaminants into the waterway.

I am no fan of AWWA valves-designed by a committee to be universally interchangabe and relentlessly average. Still, The AWWA does have widely accepted guidelines and practices and these constitute "good practice" in any juristiction I can name. If your "Safety committee" is inventing new rules and practices, they are also assuming liability. I am sure your insurance underwriter's ears would perk up if he overheard that.

 

[gerhardl]

Additional to JimCaseys remark (you are hitting the target again, JimCasey!):

Al601: Could you please expand on the reasons given for BFL valves not safe enough for the 'blocking of energy'.

If the cause for the not allowed BFLs for instance is a requirement for pressure against closed valves with a pressure peak near or above normal allowed operation pressure for the class (for instance pressure peaks by waterhammer if piumps stop); this is in my opinion off the real track.

In any case a safe 'energy holding system' would require following:

1. Pipeline structure in itself designed in pressure class for possible pressure peaks.

2. If the closed valve cannot take the pressure peak against disc or gate the valve has to be selected further pressure classes up until pressure against closing element is within allowed limit. (I have seen this done for several projects, for instance valves normally PN16 to be supplied as PN25 to hold pressure test requirements for the pipeline itself against closed disc)

3. Soft closing checkvalves (in this case non-slam nozzle checkvalves as best solution) to be installed to minimize effect of abrupt pumpstop. (Correct type of checkvalves will already be closed when delta p over the valves is zero, and prevent/minimize the waterhammer backlash)

4. Other types of necessary design adaptions to prevent and minimize pressure peaks.

For all types of overpressure and dropthight closure double eccentric soft-sealed BFL valves will be far better than the knifegate, and in this size far cheaper, especially if pressureholding requirements and dropthight closing are high.

Only weak constructed types of centric BFL types with weak or no gear will have a possibillity of be pressed out of closed position. Good constructed double eccentric valves with good types of mechanical gears (for instance slider-crank, and wether automated or manual) will not
be exposed for this.

 

[Al601]

The block I am refering to is if we had to remove a pipe spool and the only thing between the worker and the water was a butterfly valve. The potential is for injury if the butterfly shaft broke or disc slipped on the shaft the valve would fall open. Gate and knife valves if they fail (Shaft breaks) would remain in the current position.

 

[stanier]

When you remove the pipe spool fit a blind flange. This is common practice.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[gerhardl]

Normally any kind of repair on pipelines by puncturing the system by removing components will require pipeline emptied and without pressure, also before blinds are mounted. Blinds could therefore be a problem to mount downstream of a pressure sustaining valve if the valve is not qualified/certified for this.

One solution, as Al601 describes, is to install a mechanically secure valve, and if required in a double series and again if required with drainage between. (Drainage valve of top level type not to create problems, regulating valve construction best)

A mecanically secured valve could again be a double eccentric BFL valves with flanges of construction where shaft and disc are of a construction able to withstand all pressure peaks.

Take for instance a look at double eccentric BFL valves commonly used for gravity based hydroelectric plants and used as turbine inlet and emergency shutdown valves, as an example of solid constructed valves, serial produced at prices lower than knifegate and gate in this dimension. Here all construction details are documented and all strength properties certified. This type is also commonly used for water mains , and normally with mechanical gear, if the purpose is not emergency shutdown.

For turbine inlet and emergency shutdown this type of valve is often commonly supplied with oil hydraulic cylinder for opening and arm (lever) with weight for closing.

Both gear and lever can be equipped with mechanical secured locking devices, and the valve could qulify for the safety requirements.

 

[Spud6414]

We use butterfly valves all the time in plants and they control energy just fine, and you can get many with self locking mechanisms to facilitate a proper lock out tag out. I personally would like to hear your safety committee's logic behind their reasoning. The amount of space of a knife valve verses a regular gate valve is not all that much different, in the application of 24" lines that savings on space is nominal. A huge weight difference though. Knife valves have stuffing box issues as have been pointed out by others as well as areas where fluid may sit stagnant for long periods, and if it is drinking water you are dealing with, that would be a concern I suppose. I have seen gate valves fail open so that theory on safety is bunk. A knife valve would work in your application, but be prepared to teach your folks a lot about stuffing boxes.

Spud says; Meticulous effort and willingness to learn are honorable traits.

 

[gerhardl]

spud6414:

I agree, with some small additional remarks:

Sealing constructions for knife-gate valves vary in quality from producer to producer, and some of the best are not that bad on stem/shaft and top sealing of gate and U-sealing for gate seat.

The but is however still as you point out: hygienic reasons and a regular maintenance and also regular renewal with new sealings is required, standing time (say)from 1 - 10 years depending on quality and construction.

Weight differences are depending highly on pressure classes for the two types. Typical a BFL,DN600,double eccentric, flanges, nodular cast iron, with manual gear for drinking water will weigh about 330kg for PN10, and 430 for PN16.

A typical manual knifegate, with handwheel only (no additional gear) PN10 flanges but allowed for max pressure 6 bar, operational allowed pressure (thight closure both ways) will vary with supplier, lowest PN3 (nominal pressure in bar for closed gate) PN4 and PN6 (rewspectively for three different producers). Weight ranging from 340 to 370kg.

Weight for regular gatevalves though, will of course be much higher.

The conclusion is as before: go for the BFL, but high quality.

 

[Spud6414]

Agreed.

Spud says; Meticulous effort and willingness to learn are honorable traits.

 

[stanier]

The thing to consider is that some authorities, such as Sydney Water, do not consider a butterfly valve as a form of isolation in any double isolation scenario. In assessing the risk one does not always know the characteristics of a valve installed years before.

A butterfly valve is a known risk particularly where the disc is fastened to the shaft with cotter pins. The disc can rotate on the shaft if the cotter pins have corroded and sheared. Discs that are shrunk on or fitted with a spline are of a lower risk.

A butterfly valve if tested can be used as a temporary isolation for the period where a blind is inserted.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/