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[HWK77]

We have an slow closing valve in the end of a vertical pump column which does not function correctly. The float is being kneaded. We are thinking to install an orifice plate in the flange of the base of the valve to reduce water hammer.This will help to solve the problem? Or it has another solution?
Thanks for your help.

 

[BRIS]

Hwk77

I think you need to better explain the problem and the proposed orifice plate solution before we can comment.

I assume that you have a rising main discahrging into a tank through a slow closing float valve. The slow closure rate was set to limit waterhammer pressure. I don't know what the "float is being kneaded" means ??

Brian

 

[HWK77]

When the vertical pump starts, the air inside the column (above the water level) will be forced by the water pumped to escape by the slow closing valve- there is a check valve, just after the valve, retaining downstream water. The water velocity in the column depends on the velocity of air exit. We are intending to reduce the velocity of air escape with an orificie before the valve (with a minor diameter than valve). Thus the water will arrive at the float with lesser velocity and minor impact. (When the impact occurs the float is launched against the valve body being damaged.
Thanks for your attention.

 

[PUMPDESIGNER]

Could I ask a question before I answer?
What type of valve are you speaking of?
Is it ball, diaphragm type, butterfly, etc.?

PUMPDESIGNER

 

[HWK77]

It is an "slow closing valve", a type of air/vacuum valve with a float inside and a special designed chamber to release the air of vertical pumps and other necessities.

 

[BRIS]

1) First why have you got air in the vertical pipe - do you not have a check valve downstream of the pump to prevent backflow and drain down through the pump.

2) Second - the use of slow closing non slam air relief valves to reduce water hammer is common. The provision of an orifice will reduce the rate of air evacuation but it will not prevent slam when the water reaches the float so I doubt that the orifice will have a marked reduction in the water hammer pressures. SA solution is to provide an adjustable orifice (tap) on the outlet from the valve to maintain a back pressure on the valve. You will be able to adjust the tap to give optimum conditions. (see page 3 of the following link)

http://www.gavalves.co.uk/products/GA_Valves_-_Air_Valve_Catalogue.pdf

I would suggest you look at not allowing drain down and air into the system in the first place and replacement of your existing non functioning air relief valve with a proprietary non slam slow closing air relief valve.

Brian

.

 

[HWK77]

It is impossible not to allow the entrance of air.
The water column sucks air by the small passages of the packing.
I read the indicated article. The tap seems a good solution. I am verifying the possibility of installs it.
The installed valve is anti-slam, however it does not function adequately.
thanks

 

[PUMPDESIGNER]

I would have to know more to make any specific comment.
I will say this in the hope it will help you.

I was in a manufacturer's test lab where the method of choice (read cheap) for creating extremely high pressure surges was to push a column of water down a pipe with an orifice at the end, with air in front of the water column. The air escapes the orifice rapidly, but when the column of water hits the orifice pressure surges are created, easily reaching 5,000 psi. The equipment is small, cheap, and easy to make and play with. I watched parts explode under these test conditions.

Your situation is very much like that.
I would go for simple, and something at the source rather than the end. Ramp up the pump with a drive, slow opening valve (motor operated butterfly?), similar things that can be installed at the source.

If you cannot do it at the source for unavoidable reasons, why not use an extremely linear closing PLC controlled motor operated valve? Something that would be close to fail safe seeing the damage that could be done if something does not work correctly.

My company is at this moment preparing to replace a whole bunch of equipment on a job where they have been having similar problems. This is a world class project, quite famous actually, so it is surprising. But they are depending on "slow closing" valves and have suffered immensely on this issue. One year ago we proved the solution by building and installing PLC controlled motor operated buttefly valves. Actually less expensive than what they had already installed. They are so happy after proving it for a year we are going to replace all their current five year old systems. I think this is tragic in a way, to install top notch "slow closing" devices and then suffer for it?

Actually, I have learned to consider the words "slow closing" to be an indication of trouble, big trouble. Any manufacturer using those terms is either ignorant of the problem (most common), or just made a simple understandable mistake in terms. The issue is not one of "slow closing", that is a red herring, not relevant.

The issue if LINEAR CLOSING and CONTROLLED RATE OF CLOSURE.

What say others on this?

PUMPDESIGNER

 

[hacksaw]

You might consider use of a continuous recycle, some times referred to as a minimum flow bypass. The float type air release valves were never intended for cyclical operations on pump discharges like that.

I agree with P/D that true control valves are a better solution and one that works.

 

[smckennz]

If your "slow closing" valve is a float controlled air vent, then an upstream orifice plate will work just fine on pressure rise. You also need to consider what will happen when the pump turns off. It may be necessary to let extra air in via a check valve, or something fancier, when the pump stops.
In fact you may want to review the entire system hydraulics as the existing valve may not be necessary if the air can be carried away with the water.

It may be worth considering that if the float is being deformed (kneaded?), then you have some fairly serious pressure spikes to deal with.
The normal ways of dealing with them are:
1) keep some air in the system to cushion the impact. This is what your orifice plate will do.
2) Stop the pressure rising so fast. This is what a pump control valve, and, to a lesser extent, a fixed bypass will do. Alternatives are variable speed drive ramp start and soft starter. With the soft starter you will find that everyone says they are great at the beginning, and then they get evasive when you request finalised speed/time information, so be careful before committing.

And it is normally best to have the check valve close to the pump discharge; not at the top of the line. Reduces vacuum hammer problems, and will reduce the amount of air getting in as the riser will not be under suction when the pump is off.

Cheers

Steve McKenzie