Momentum forces in drainage pipe?

[JoacimEurocon]

Hi,

When analysing a pipe according to norms such as EN13480-3 or ASME 31.3, we generally evaluate for loads due to weight, internal pressure, thermal expansion. Other specific loads are often also evaluated for. However, I wonder why loads due to the momentum of the fluid are not accounted for by these norms. Intuitively it seems like the are not always neglectable.

To be more specific: I am analyzing a pipe which is used to drain fluid from a vessel to another vessel at a lower elevation. I.e. the pipe is empty until a valve on the upper vessel is opened and then liquid is drained through the pipe to the lower vessuel only due to gravity. The pipe has been in use for many years but the customer have seen some damage to both piping and its supports. The customer believes that the momentum of the fluid may cause excessive forces on the supports, which does not seem too unreasonable too me.

Do you guys have any methodologies or guides to share for problems such as this?

Best regards,
Joacim

 

[robyengIT]

IMHO dynamic effects due to flowing fluid (i.e. forces on bends increased by intermittent operations, that is fatigue)

 

[LittleInch]

Well you two issues here to look at.

The first is the initial transient surge force as the water rushes down the pipe and creates forces on elbows, tees etc as the fluid hits it.

This can be really quite violent as the velocities can be very high (> 10 m/sec) and also creates shaking as the air is driven out, but in the initial wave it can be a mixture of fluid and water.

Once the system has settled down, unless velocities are prob > 5m/sec, and the flow is steady, then the forces are usually quite low and with welded pipe, the axial force is small.

Push fit pipes on the other hand can separate if not contained by either axial restraints or anchor blocks / restraints.

So first you need to understand how the flow is introduced, what level of flow control is there and what sort of velocities and type of fluid motion you are seeing at the start. The same applies for the steady state flow.

To avoid this it is probably more common for the interconnection pipe to be closed off at the lower end and left full of fluid to avoid these high impact transient events or for the filling of the pipe to occur in a controlled fashion by closing the end valve and slowly introducing fluid before letting it rip....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JoacimEurocon]

Thank you for your replies @robyengIT and @LittleInch. Your explanation, LittleInch, gave me some deeper understanding to back up my intuition.

My conclusion is that this problem is generally not covered in calculation norms as you will use different engineering solutions to not fill your pipes so violently. This makes sense. However, as this is a drainage pipe that relies on gravity drain a boiler intermittently several times a day this can become a real problem, which we have to consider.

Best regards,
Joacim

 

[robyengIT]

if this is a blow-down pipe and the boiler is in pressure, I am not shure the discharge is by gravity

 

[LittleInch]

Then you maybe need to restrict the inlet flow whilst filling to prevent excessive velocities.

The devil is in the detail here and we don't have it...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JoacimEurocon]

Thanks again for your replies. I'm not looking for more detailed help at this moment. Your help has definately nudged me in the right direction so I can carry on with my evaluation.

Best regards,
Joacim

 

[saplanti]

If I understood correctly, the flow you are talking about is two phase flow. Pressurised and very hot water becomes steam in the pipe or during the entering into the blow-down vessel. It is violent flow, therefore its pipe to be designed to enter the vessel tangentially for most cases.

You need to talk to an expert in case our assumption is right. They need to find a solution to eliminate vibration by using adequate supports and watch the thermal expansion of the pipe. In most cases in the design office these pipes due to their sizes are left to the designer to provide support by going through a procedure, but not always come with right answer.

 

[LittleInch]

I agree. If you have a flashing liquid here then it's several times worse than just an empty pipe you're flowing some sort of liquid through.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[robyengIT]

usually the blow-down pipe ends with a drilled short pipe, bigger diam then the discharge pipe (retired, just starting to learn how to draw in 3D : that's why I modified the sketch

 

[FacEngrPE]

Avoid two phase flow if possible. Two phase flow will quickly become a stream of high velocity slugs, with resulting hammer at any change in direction. On larger diameter pipes the forces at elbows can be high enough to punch out of the piping system.

The National Board of Pressure Vessel Inspectors issued this document which has some recommendations that work for boiler blow down pipes. NB-27: A Guide for Blowoff or Blowdown Vessels. This document is out of print, so it is attached.

The appendix has some pipe sizing information that works for boiler blow down pipes. This is a guidance document, not a "Code".