Check Valve slamming, Soft start for de-acceleration?

[Mule]

A frequent customer just called and has a 7 1/2 hp water pump with a 3 inch discharge that goes up a hill. He is claiming that when he shuts the pump OFF the check valve is hitting way too hard from the head pressure. He's been talking to a mechanical person, and thus is now suggesting, and asking me about installing a soft start. He claims that by eliminating the friction by adjusting down the de-aceleration on the soft start they can soften the water hammer on the line.

Im think it wont work because the weight of the water is still there and moment that the impeller stops reguardless of the de-acceleration time, here comes the weight trying to back spin the impeller, thus slamming the check.

I suggested installing another check half way up the hill, or a air bottle just after the check, and also suggested that having a soft start "out in the sticks" really close to the power line could possibly be more of a reliability issue with lightning strikes and surges.

any comments, apreciated.....What's the real fix?

 

[charlie b]

I talked with one of the plumbing engineers in our office, a man whose professional opinions on plumbing issues are widely respected. His answer: “Tell the customer to live with it.” He specifically addressed the two suggestions you mentioned. He confirmed my suspicion that a soft start would have no effect. He surprised me with the statement that a second check valve uphill would have no effect.

He said that the physics of this situation are simple. Once you have enough differential pressure across the check valve, it is going to close. If the hill is more than, let us say, ten feet high, then nothing can make the valve-to-seat contact less violent. A bit of insulation might help dampen the noise, but that's about all that can be done.

I didn’t ask him this, but I suspect that a “better” check valve might be less noisy. Something sturdier, or perhaps something with different materials of construction, might give the owner less to worry about. If indeed the existing check valve is a cheap and poorly constructed model, then there may be a real concern over premature failure, especially if it was not intended (by its manufacturer) for a hard-usage environment.

Bottom line: this is a mechanical issue, and cannot be solved by an electrical means.

 

[iwire]

They do make dampened check valves, don't know the cost but likely less cost then the VFD.

 

[Cold Fusion]

I talked with one of the plumbing engineers in our office, a man whose professional opinions on plumbing issues are widely respected. His answer: ?Tell the customer to live with it.? He specifically addressed the two suggestions you mentioned. He confirmed my suspicion that a soft start would have no effect.

Bottom line: this is a mechanical issue, and cannot be solved by an electrical means.

charley -
I'm assuming both you and your hydraulic whiz are familiar with Allen-Bradley SMC 150, pump control, start and stop. It's a little over half way down on the link.

http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/1203377/tab3.html

I recall putting in whatever AB was selling in the mid-80s that was the fore runner to the SMC 150. It was on a verticle shaft, 60hp with a submerged centrifigal pump, pumping from a waste water tank to a treatment unit, 200 feet of head.

When the motor was shut off, the pump would spin backwards for a second then the check would slam shut. It would shake the piping pretty good.

The mechanicals had me put in the AB soft start, program for the pump control, and tune the soft stop to minimize the hydraulic shock.

Is this the type of thing your hydraulic whiz is saying won't work?

This was 20 plus years ago and I don't remember how well it worked. However, since I also don't remember any screamng, nashing of teeth, or being pummeled by the mechanicals for the #*@#*&% electrical not working, I suspect it must have done something they liked.

cf

 

[Cold Fusion]

Mule-
Is this the 7.5hp on the POCO provided, ungrounded, 480D?

Likely doesn't matter for this post - just curious.

cf

 

[Mule]

I talked with one of the plumbing engineers in our office, a man whose professional opinions on plumbing issues are widely respected. His answer: ?Tell the customer to live with it.? He specifically addressed the two suggestions you mentioned. He confirmed my suspicion that a soft start would have no effect. He surprised me with the statement that a second check valve uphill would have no effect.

He said that the physics of this situation are simple. Once you have enough differential pressure across the check valve, it is going to close. If the hill is more than, let us say, ten feet high, then nothing can make the valve-to-seat contact less violent. A bit of insulation might help dampen the noise, but that's about all that can be done.

I didn?t ask him this, but I suspect that a ?better? check valve might be less noisy. Something sturdier, or perhaps something with different materials of construction, might give the owner less to worry about. If indeed the existing check valve is a cheap and poorly constructed model, then there may be a real concern over premature failure, especially if it was not intended (by its manufacturer) for a hard-usage environment.

Bottom line: this is a mechanical issue, and cannot be solved by an electrical means.

That's interesting info....thanks...it gives us another thought path for sure....
So I tend to now agree that the check is going to close at a specific designed pressure differential before, and, reguardless of the maximum head that is ultimately experienced downward. I called the customer and read it to him on the phone, he's in a bit of disagreement thought, but is spining it around in his head.

This is a great forum..........

 

[Mule]

Mule-
Is this the 7.5hp on the POCO provided, ungrounded, 480D?

Likely doesn't matter for this post - just curious.

cf

Your good, YES it is....

 

[Mule]

charley -
I'm assuming both you and your hydraulic whiz are familiar with Allen-Bradley SMC 150, pump control, start and stop. It's a little over half way down on the link.

http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/1203377/tab3.html

I recall putting in whatever AB was selling in the mid-80s that was the fore runner to the SMC 150. It was on a verticle shaft, 60hp with a submerged centrifigal pump, pumping from a waste water tank to a treatment unit, 200 feet of head.

When the motor was shut off, the pump would spin backwards for a second then the check would slam shut. It would shake the piping pretty good.

The mechanicals had me put in the AB soft start, program for the pump control, and tune the soft stop to minimize the hydraulic shock.

Is this the type of thing your hydraulic whiz is saying won't work?

This was 20 plus years ago and I don't remember how well it worked. However, since I also don't remember any screamng, nashing of teeth, or being pummeled by the mechanicals for the #*@#*&% electrical not working, I suspect it must have done something they liked.

cf

So Im starting to visualize that the spring pressure of the check design is essential to this problem....needs to close at a smaller differential difference..

I know this is not electrical in nature, but is interesting...sorry if Im bending the rules

 

[charlietuna]

Yes it should work since you can set the de-acceleration of the motor slow enough to have very little pressure drop across the check valve as it closes. We had a similar problem in a plant that pumped liquid expandable foam into large boat forms -- so that the exact amount had to be ejected--too much would cause the form to explode..... The engineers were not very good--boy did they go thru a lot of forms!!!

 

[ohm]

charley -
I'm assuming both you and your hydraulic whiz are familiar with Allen-Bradley SMC 150, pump control, start and stop. It's a little over half way down on the link.

http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/1203377/tab3.html

I recall putting in whatever AB was selling in the mid-80s that was the fore runner to the SMC 150. It was on a verticle shaft, 60hp with a submerged centrifigal pump, pumping from a waste water tank to a treatment unit, 200 feet of head.

When the motor was shut off, the pump would spin backwards for a second then the check would slam shut. It would shake the piping pretty good.

The mechanicals had me put in the AB soft start, program for the pump control, and tune the soft stop to minimize the hydraulic shock.

Is this the type of thing your hydraulic whiz is saying won't work?

This was 20 plus years ago and I don't remember how well it worked. However, since I also don't remember any screamng, nashing of teeth, or being pummeled by the mechanicals for the #*@#*&% electrical not working, I suspect it must have done something they liked.

cf

I'll bet it's working just fine, perfect fix.

 

[don_resqcapt19]

If you can recycle the water at the pump you could use a 3 way auto valve before you stop the pump. When the pump is running the valve would send the water up the hill. When you want to stop the pump you first operate the valve. It would open to the recycle line and close to the hill. Then you would stop the pump. There would be no water hammer.

 

[electricman2]

I have seen this setup used with sucess on high head situations: A motorized butterfly valve is used in conjunction with the check valve. The butterfly valve closes while pump is still running. Motor stops when valve is near closed. This prevents the hydraulic shock of the sudden check valve closing.

 

[TOOL_5150]

what if a T was installed right after the valve [load side lol] and a verticle air chamber was installed... Since air compresses 5 times as much as water, it would dampen the blow to the valve.

~Matt

 

[ceb58]

We have several lift stations that are in the same situation pumping up hill. Now given these are 25hp pumps with 6" lines. The check valves that are installed have springs and counterweights mounted on the outside of the valve body. when the pump shuts down the spring and counter weight causes the flapper to close slowly against the back pressure and doesn't allow the flapper to slam shut on the seat.

 

[Mule]

So after reading all of these post, sounds like check valve spring pressure, or adjusting the de-accel will allow some control of the check slamming....I'll relay the info to the customer, thanks

 

[charlietuna]

As an example if you set the decel at three minutes--don't you think the valve would seat quitely as the pump reached near zero gpm ?

 

[ptonsparky]

Several of the village wells in the area use water pressure from the lines to open the check valve as water being pumped hits the valve, then releases this pressure to close it. When timed and plumbed right you don't even notice them working.

A VFD is a lot cheaper for this installation.

The check valve will seat once the pressure on the outlet side exceeds the pressure on the inlet and long before the pump reaches 0 GPM.

 

[Fulthrotl]

A frequent customer just called and has a 7 1/2 hp water pump with a 3 inch discharge that goes up a hill. He is claiming that when he shuts the pump OFF the check valve is hitting way too hard from the head pressure. He's been talking to a mechanical person, and thus is now suggesting, and asking me about installing a soft start. He claims that by eliminating the friction by adjusting down the de-aceleration on the soft start they can soften the water hammer on the line.

Im think it wont work because the weight of the water is still there and moment that the impeller stops reguardless of the de-acceleration time, here comes the weight trying to back spin the impeller, thus slamming the check.

I suggested installing another check half way up the hill, or a air bottle just after the check, and also suggested that having a soft start "out in the sticks" really close to the power line could possibly be more of a reliability issue with lightning strikes and surges.

any comments, apreciated.....What's the real fix?

water hammer stop uphill after the check valve.

souix chief are about the best, these work well, and here is the link
for correct sizing.... pipe size, head pressure, working pressure,
there are a number of considerations.

http://www.siouxchief.com/Frm_MS.cfm

 

[charlietuna]

Your customer may also consider the VFD as a pressure regulator where it ramps up on demand--this could save him a lot of money. We have retrofitted high rise buildings and cut their water pump operating cost in half?