Thermal Expansion-Bladder Compensators

[RJB32482]

My question is on thermal expansion compensators that have bladders precharged with N2 on the unwetted side. What is the hazards with having the charge too high or too low of a pressure relative to the normal operating pressure of the system? Would too high cause not enough volume to go into the compensator? Would too little cause the compensator to fill even while pumping (if compensator is vertically mounted and no valve on the bottom while pumping through main lime)?

Any learning would be helpful.

Thanks.

 

[djack77494]

I think you answered your own question. Yes, too high a pressure will reduce the effective volume available to absorb the expansion. Too low a pressure and much of your "active" volume will normally be filled with liquid. Again, not available for the intended purpose. I am puzzled however with your description of the problem. I have used expansion bottles, bladders, compensators, or whatever you'd like to call them on numerous occassions, but never for the purpose of "thermal expansion". (I assume your fluid is a liquid.) Very small expansion volumes can handle the "normal" expansion of a large volume of liquid. If you'd like to more fully describe your problem, I'll attempt a more complete answer.
Doug

 

[RJB32482]

Thanks for the reply.

We have a styrene line that feeds styrene through a pump during normal operations. When its not running, there is a blocked in section of pipe that can be heated and expands. If a expansion dampner is put in, this would help to decrease the pressure in the line.

Thanks.

 

[djack77494]

A pretty general approach would be to postulate a realistic but "worst case" scenario in which the stagnant liquid is initially at its lowest likely temperature. Now input heat at the maximum rate you can. (I assume you have steam or electric tracing or some other means of inputting heat). Start with 100% of the liquid filled volume. As it now heats, it expands. You want that expansion to compress the gas you charged in the expansion bottle. You can use the cubic expansion formula to go from an initial to a final temperature to figure out the volume of liquid that you want to move into the expansion bottle. You could do the same thing if you know the density of the liquid at the two temperatures. Design the expansion bottle to handle a healthy amount beyond what you've calculated (overdesign). For 100% liquid, the volumetric change tends to be very small, but it's a good exercise to do.
Good luck,
Doug

 

[sshep]

djack,

Remember that when the bladder is in service, the N2 side pressure is the line pressure (when line pressure exceeds charge pressure). If this is a user fillable system, the process side should be depressured and the bladder pressure checked periodically (annually). We had one burst due having the N2 charge slowly leak away leaving no room for thermal expansion.

This servicing may require an isolation and bleed valve if the protected piping cannot be depressurized, but I feel these devices still offer some advantages to thermal relief valves for areas where there is no satisfactory discharge location. We have used in tank farm areas, etc.

best wishes,
sshep

 

[RJB32482]

sshep,
Why would the N2 leaking away into the process cause no room for thermal expansion? The N2 gas still should compress in the tank, allowing the volume of a liquid to rise in the expansion tank.

Why is the bladder even needed in an expansion tank of this sort? Why not just have a N2 blanket on the vessel lower than the normal pressure of the system.

Thanks.

 

[sshep]

RBJ,

Answer#1, If N2 is leaking into the process then you have a hole in the bladder and your accumulator is in need of a replacement bladder (if a reparable type), however, the leak I was talking about is a slow leak out the fill stem and to the atmosphere. Envision a completely flat bladder inside an otherwise totally liquid full vessel- i.e. expansion capacity is zero. In any event some regular checking of these devices is required.

For the type of device I am familiar with see (has plenty of sizing data for different applications):

http://www.fluidpower.com/Accumulators/Products/products.html

Answer#2, there are vertical mount devices that don't use a bladder, but they generally need a float type valve at the base to prevent N2 leaking into the process during low process pressure. Using an atmospheric pressure initial charge for a bladder-less accumulator would require a vessel too large for many applications. Also some N2 would dissolve away over time requiring the at least the same (but probably more) frequency of preventative maintenance as bladder types.

best wishes,
sshep

 

[djack77494]

sshep,
I agree with your comment and didn't mean to suggest otherwise. You must figure out the useful (available for expansion) volume of the expansion bottle and use that and not the complete volume of the bottle in your calculations. Since it is easy to err in the calculations and since there is a bit more to this technology than may be apparent, I'd always get a "reality check" from the vendor. Also, always have a working pressure gauge on the nitrogen side of the bladder. It will help with troubleshooting (which is mainly determining if the bladder is leaking). Incidentally, some of these devices use pistons rather than bladders.
HTH,
Doug

 

[sshep]

djack,
When I addressed above post, I actually mistook you for the original poster (RBJ). Your comments are right on. Thermal expansion can be analyzed by simple "isothermal" compression of the gas in the bladder, which is easier calculation than a pulsation dampener application where adiabatic compression is the more appropriate basis.

In addition to your recommended gage on the bladder side, it is also recommended to include a isolation valve between the bottle and the process, and a bleed. The routine preventative check is to isolate, bleed down process pressure, and check the bladder pressure. Incidently I see that my fluidpower link recommends an inspection frequency of 6 months.

best wishes,
sshep

 

[RJB32482]

Never seen the piston type of thermal expansion compensators? Any more info on them??

Thanks

 

[unclesyd]

Here are some piston type hydraulic accumulators. We used this type on a very large hydraulic press.

http://www.accumulators-hyd.com/hydraulic-hydropneumatic-accumulators/piston-accumulators.htm

Look at the information on accumulators at this site.


Look around at the links and associations on this site as there is wealth of information. This include Pulse Guard, Liquid Dynamics, Shock Guard , and several others. The only problem is that it's a little convoluted.