piping fluids 8

[coffmarm]

I have a pressure regulator set for 140 psi supplying water to a heat exchanger with a maximum pressure rating of 150 psi. If someone rapidly closes a butterfly valve downstream of the heat exchanger, will the heat exchanger see a pressure spike in excess of 140 psi? If so, I can prevent it by putting a gear-drive operator on the valve, but I'd rather save the expense if I can.

 

[JohnBreen]

What is the relief valve that protects the exchanger set for?

 

[coffmarm]

Relief valve is set for 150 psi.

 

[BigInch]

You will (almost certainly) be able to easily go well over 150 psi. The relief valve and piping must be sized for full surge flow (in case your gear op valve fails).

http://virtualpipeline.spaces.msn.com

 

[jistre]

It seems to me that the pressure regulator upstream of the exchanger will only regulate the pressure coming from that direction.

If the valve is downstream of the exchanger and it's slammed closed, then the pressure pulse is coming from the other side of the exchanger and must pass through the exchanger before it hits the regulator, so it's inconsequential to what the exchanger sees in this situation, even if it works to regulate pressures bi-directionally. If there is no relief device between the valve and the exchanger, then I'd think that the exchanger could see extremely high pressure spikes if that valve is suddenly closed.

If there is a relief device, then the pressure that the exchanger would see would be affected by the type of device it is, its set points, and the operating characteristics of the device (for instance, does it begin opening at 150 or is it fully open at 150?).

Of course, I've been wrong before.

 

[BigInch]

You're not wrong this time.

http://virtualpipeline.spaces.msn.com

 

[coffmarm]

Sounds like installing the gear operator on the butterfly valve would be a good investment. It makes sense that the slammed valve would be the starting point for the shockwave which would be unaffected by an upstream regulator. Thanks!

 

[sailoday28]

Other than the settings and locations of regulators,relief valve, etc. the pressure response of the heat exchanger will depend on where the pressure wave initially reflects to. That is --the shell side or tube side or what ever the physical make up of the exchanger is.

Depending on the downstream initial velocity, that piping will see a surge, but then again, the heat exchanger respose will have to be modeled based on its construction. At least communicate with the heat exchanger vendor.

Regards

 

[coffmarm]

Thanks. I also found a good reference on waterhammer causes and prevention at:

http://www.pdionline.org/PDI-wh 201-2006.pdf

 

[stanier]

Conventional relief valves will not always respond quickly enough to prevent a transient pressure impacting on the equipment.

A pilot operated relief valve withthe pilot piped to close to the source of the surge can give better results.

Control of valve closing speed can be effected by use of a gerarbox. beware that a butterfly valve 's characterisitcs are such that at even 10% open there could be a large flow.

An analysis of the transients conditions would enable you to design the solution. The magnitude of the pressure excursion may fall within the allowable pipe stress conditions of ASME B31.3, the heat exchanger TEMA standards and ASME VIII code.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[coffmarm]

Yes, I was planning to take some pressure readings after STS-120 launches. I'm trying to locate a fast-acting pressure gauge that will retain the peak reading.

 

[jmw]

You could investigate an accumulator as a means to absorb some of the peak pressure.

JMW

http://www.ViscoAnalyser.com

 

[stanier]

Coffmarm,

Your original posting mentioned saving the "expense". I am always fearful when that sentiment is expressed.

As professional engineers we have to manage costs as well as technical matters. Ultimately what we design must work safely and hence cost is secondary. Remember the cost of failure is far higher than the correct solution. Engineers go to jail not project managers.

Some good reading on the European Directive of Safety is available on the web. So if you do business in Eorupe there are very stringent requirements.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[coffmarm]

My company's priorities are safety, schedule, & cost - in that order.

 

[stanier]

Great to hear.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[bduane]

If the valve closes faster than (2*(pipe length))/(speed of sound in the medium) it is considered an instantaneous valve closure and the pressure rise (above the operating pressure) is easily calculated by the formula in most engineering texts on fluid transients.

If an accumulator is used it needs to be sized and charged appropriately or it may not absorb the pressure spike and is some cases can make it worse.

This problem is easily modeled w/ commercial software packages for surge i.e. Hammer, KYsurge, and Impulse to name a few.

The response time of any relief device is critical to protecting the equipment as indicated above.