Calculating Line/Pressure Loss Across Expansion Joints 5

[carltogr]

Guys & Gals --

Does anyone know how to calculate the pressure drop across an expansion joint? Or, what kind of pressure losses and/or K (Cv) values can one expect from an expansion joint? Lets say for water at any condition. I have checked with several manufactures, and they don't seem to test for this property....

 

[CHD01]

I would used the minimum ID of the expansion joint as a safe estimate; but that is just my opinion. Typically I do not even bother to break this out for analysis of pressure drop. I would not be concerned unless determination of pressure drop was extremely critical. Anyone out there with an experience where such detail was justified?

The more you learn, the less you are certain of.

 

[25362]

The Pipe Friction Manual of the Hydraulic Institute gives the resistance coefficients in feet of fluid for friction through sudden enlargements as well as for gradual diameter increases and diffusers.

 

[friartuck]

If by expansion joint, you mean a flexible hose??

Some expansion joints are lined and so the loss will be small. Other joints may have corrugated inners and create a higher loss.

If the question was for flexible air ductwork hose, the PD is about 2 or 3 x the rate of standard ducting (for corrugated compared to straight).



Friar Tuck of Sherwood

 

[rmw]

Like friartuck, I would like a better definition of the type of expansion joint we are considering.

In many types of duct and piping EJ's, there is a flow liner built into the joint to protect the convolutions (metal) or material (fabric) of the joints.

Flow distrubances that would contribute to pressure drop in the system would be specifically related to the protrusion (or not) of the flow liner into the flow stream.

Joints with no flow liners, now, that is another matter.

rmw

 

[sailoday28]

I don't have a feel for how far the protusions are into the main flow field. If not very large, consider the Moody friction diagram and let the protusion depth represent the epislon part of epsilon/Diameter. Having a Reynolds number would then give you a feel for the DP. I suspect that the length portion of fL/D will help reduce the severity of the value obtained from the Moody diagram.

 

[carltogr]

It is a Teflon lined expansion joint with bellows -- very much similar to the Unisource, 1501 TN (PTFE lined), or similar to the Mason (Mercer) Industries 700 series. Hope this helps....

thanks

 

[Tremolo]

Carltogr,

The reference “Idelchik, I. E., 1986, Handbook of Hydraulic Resistance, Second Edition, Hemisphere Publishing Corporation, New York (1986)” is the ultimate source for obscure loss coefficients.

Page 462 of Idelchik has loss coefficient for expansion joints. The loss coefficient depends on the type and size of expansion joint. I can e-mail you the page from Idelchik that discusses expansion joints. Please contact me at Elicson@fauske.com.

For bellows type expansion joints, Idelchik reports K=1.7 for 50 mm diameter bellows up to K=2.3 for a 500 mm diameter bellows.


Hope this helps.

 

[Procman]

Hi Tremolo

I'm looking for the k values for friction loss thru' expansion bellows for a 150mm and 200 mm ID pipe. Please could you help. Will try to get a hold of this handy reference

 

[Tremolo]

Over the weekend my computer crashed and it is now in the repair shop, so I don't currently have access to the Idelchik reference.

I would appreciate it if anyone who I e-mailed the reference to could e-mail a copy of it back to me at elicson@fauske.com.

Thank you.

TREMOLO

 

[Procman]

Oh Cricky. Well. there's very little room to move between a K=1.7 and K=2.3...would I be too far out if I interpolated?

 

[Tremolo]

Procman,

As I recall, the loss coefficient increased monotonically with increasing pipe diameter. So, interpolation should give you a reasonable estimate.

This is what I get for trying to store everything electronically and eliminate paper from my office. I've failed on both accounts - my electronic files are inaccessible and my office is still overflowing with paper.

TREMOLO

 

[iainuts]

There was a paper published in the early '60's in which the authors experimented with convoluted metal hose, such as is used for expansion joints. The restriction of the metal hose was correlated to an equivalent length of the same diameter pipe as follows:
Input:
Minimum ID (ID) (inches)
Length (L) (inches)
Number of corregations per inch (N)
Roughness factor (e)

Output:
Fully Turbulent Friction Factor (f) = 1/(4*(0.57-LOG(12*e/ID))^2)
Equivalent Length = L*N*ID/f*(1-(ID/(ID+0.438/N))^2)^2

You should find the equivalent length of a section of convoluted bellow is generally between 8 and 12 times the actual length.

 

[25362]

The Plant Notebook of the CheE issue of August 4, 1986 brings graphs for a Quick determination of resistance coefficients for reducers and enlargers by Ramsay Nokay.

 

[johnpe]

Iainuts friction factor formula is a bit tricky. I think foot and inch dimensions are mixed up. If cocnsistant units are used the term (12*e/ID) should be replaced by (e/ID) where e is the roughness hieght. This formula duplicates the Moody diagram beautifully.
A bit of algebra converts the equivalent length formula to one for the friction factor for the corrugated hose:
f=N*ID/{1-[ID/(ID+.438/N)]^2}^2
If this formula is correct using the height of the corrugations as the roughness hieght in the Moody diagram, as sailoday28 suggests, overestimates the friction by a factor of 2 (at least for the 6" size that I'm interested in).