How to determine allowable rapid temperature change in piping at our plant? 7

[USAeng]

Good morning all. We are looking at using a thermal oil for heating some equipment at our plant. The oil is kept at 350F in a main building and is currently used for one of our processes.

The piping to the new equipment is run outside and is insulated. Can someone point me in the right direction on how to determine if we are OK with pumping this hot oil into a potentially cold pipe? We are located in Ohio where temps get down to about possible -20F.

We want to provide verification that we do not have to spend electric cost to constantly circulate the hot oil through the piping to maintain temperature in this heating loop.

Here is piping specs

Thermal fluid piping shall be seamless carbon steel ASTM A53 Gr B standard wall
All pipe flanges shall be forged carbon steel ASTM A105, ASME B16.5, Class 300 welding neck, raised face, with Std WT bore

If anyone is curious about the oil it is Paratherm HE - specs attached

We use spiral wound gaskets on all flanged connections. Pipe size is 3"

If anything is unclear let me know. Thanks

 

[BigInch]

You must confirm that inside stress at (350--20)F * [α]*E are less then allowable tension stress of the outside fibre (at -20F). Inside fibre contained expansion stress is likely to be around 75,000 psi. Looks like you must keep the maximum in-outside temperature differential much lower than the 375F you could have for an un-insulated, cold pipe.

 

[USAeng]

BigInch - thank you for this reply

OK after searching we find E to be about 29.5x10^6 PSI and coefficient of thermal expansion to be about 6.5x10^-6 in/in°F.

The yield strength of the piping is 35,000PSI so do I divide the 35,000 by the multiple of the numbers above to get max temp differential in the ballpark of 182°F?

 

[BigInch]

That's just the thermal ring expansion stress. Does it not also have pressure.

 

[USAeng]

Yes there is 125 PSI in this line

 

[georgeverghese]

At -20degF or -30degC, would brittle failure concerns require some piping material other than A53 at the corresponding upper design pressure?

If you find the thermal shock load (dT/dt) at -20degF on the piping is in excess of what is permissible, would electric heat tracing of these pipes be a lower operating cost?

 

[BigInch]

I wouldn't think immediately that tracing is needed, unless it must be shut down during cold weather and the pipes can't be drained, otherwise only a slower warm-up time using lower temperature oil initially, then slowly increasing oil temperature may be all that is necessary. As long as the in/out temp differential is limited to keep warmup stresses low, it should be fine. I take it that this pipe does have insulation.
The A53 pipe itself can work at these lower temperatures, but impact testing should be specified in the PO to be conducted at whatever the lowest temperature is, or test even lower at lowest temperature -5 additional degrees F or so.

 

[georgeverghese]

What if you kept this outside line flooded with hot oil, and used a 3/4 inch needle valve on a bypass line to flow a small amount of hot oil through this line all the time to keep it warm ( say at 30degC or so ?), so you wont need to trace it. And at 30degC, heat loss through the insulation would be next to nothing. You could use this needle valve for startup-warmup to 300degF also.

If this is acceptable, you'd still need to account for impact testing requirements in case Operations have somehow missed out on keeping this line warm.

Just took a look at this fluid's physical props - its off the curve on viscosity at 30degF !

So, what happens at -20degF if for some reason this trickle warming operation was missed out - would this liquid freeze up? No data on freeze point, and viscosity of 320cP at 30degF is worrying enough. Having to drain out this line everytime you shutdown relies on operating procedures also.

Looks like external electric heat tracing (obviously not steam tracing) is the only way to cold start to warm up reliably with this Paratherm. A low temp alarm at control room should be included.

A piping engineer should suggest what dT/dt is permitted at 30degC or so, when you begin warm startup operations with the bypass needle / globe valve.

 

[europipe]

You have to run a pipestressreport on the line in cold and hot conditions.

 

[LittleInch]

good spot from George - that sort of viscosity will be close to gel like at -20F.

My issue with this is that you seem to be looking for a static equation for a transient event - never normally a good way to do things. I also can't see your system so have no idea how long or how convoluted this system is. My concern would be that one section of the pipe would be heating up and expanding, whilst the other is still cold and with lower strength properties. This is very difficult to model accurately.

I think the way out is to maintain some sort of temperature in the pipe (say 30 to 40C) and then re-start over a period of 5 to 10 minutes at a low rate. You should be running this through a transient stress model / transient flow model to show temp rise over time taking into account metal thickness and heat p time as well as the hot oil.

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

 

[BigInch]

Yes that's a good one George. That line should not be allowed to go low temp.

 

[USAeng]

Thank you all for kind responses

We do not have transient flow software and even if purchased would not have the experience necessary to correctly use it. We may consider subbing this out unless we can determine some other way

For certain we must keep the fluid temp above a chosen number to allow pumping - we can do this

I attach a simple P&ID so you can see more what is happening. There is much more obviously but I do not think it mattered to include for the discussion

The length of piping is about 500' of 8" pipe for the main dryer loop as shown in attachement. The secondary loops (EET loop) have between 100 and 300' of 3" piping depending as there are 3 loops. There are some transitions with stepped down sizes to the 3" and back to the 8" but not shown

 

[USAeng]

Flow rates in the line are approximately 900GPM in the main line and 125GPM in each EET loop. No speed control on the pumps

 

[Gator]

This is of no help at all: your pumps are drawn upside-down and the strainer is drawn backwards. Who care though?

Piping Design Central

 

[USAeng]

Gator, I am not experienced drafting. I appreciate you pointing out these items. Thanks

 

[Compositepro]

While sudden temperature changes may cause cracking if severe enough, fatigue cracking is more common with thermocycling. This occurs at much less than ultimate stress. A mitigating factor is that the heat transfer coefficient from oil into the metal will significantly lower the actual delta T from inside to outside of the pipe.

The drawback of using a restrictive orifice to keep the pipe warm is is that the pipe will still be 350F at one end while the other end will be far colder. You should have a separate pump to circulate oil in the new heating loop and slowly introduce hot oil into this loop with appropriate controls. Just the heat of pumping will tend to keep the fluid from getting too viscous because the heating will increase with viscosity. Thus the temperature will tend to be self-regulating.

All the discussion so far has dealt with the hoop stresses in the pipe due to rapid temperature change. In most cases dealing with the the longitudinal stresses cause by a large temperature change and anchor points are the greater challenge.

 

[BigInch]

True, provided that the pipe survives warm up.

 

[USAeng]

Thank you all for your thoughtful comments. Most likely we will just keep all 3 pumps running full time. We will also look into some temp based control scheme but will likely just keep them all on. Thanks again

 

[georgeverghese]

There are limits to dT/dt for recirc pumps also, which would apply for these secondary heating loops - this may mean that "trickle flow" may be interpreted otherwise as running a fixed speed pump almost on full recycle so that net forward flow is just the warm up trickle flow - the pump discharge valve may need to be throttled temporarily also to get to this flow. Pump vendors seem to require very low mechanical loads on pump nozzles, and thermal expansion of these pipes may result in having to install expensive CRA expansion bellows that will not corrode externally if chloride SCC is applicable at this location. If this Paratherm hot oil is flammable at the operating temp, plan 52 pump shaft seals would be a safety requirement as a minimum - hot oil pump fires originating from leaking shaft seals are a common occurence.