Am I at risk of a steam explosion? 2

[cdill21]

Hello All,

I am trying to see if I have a risk of a steam explosion.

I have taken over a legacy product line which is effectively a small (0.5 ft^3) water heater on wheels for industrial use. In this particular unit, I have observed a 30,000 btu/hr electric heater in the system, with a 15,000 Btu/hr 1/2" relief valve.

This is not a passive heater system. It is designed to flush the heater tank with incoming cold water, more than sufficient to dump excess heat from the heater and keep the water from boiling until the operator notices and shuts down the machine. The failure mode is a stuck mechanical heater contactor. The tank itself is designed to a burst pressure of 2000 psi, but as I understand it, CSA only considers systems with a MAWP of 150psi.

In the event of a failure for the system to flush cold water, I am confident that the current pressure relief valve is sufficient to continue to dump steam until the heaters burn out, before the pressure vessel would burst.

Is this a reasonable assumption? T

 

[EdStainless]

Has it ever been tested?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[IRstuff]

2256.4 kJ/kg heat of fusionvaporization at 100C x 0.5 cf x 1 gm/cc divided by 30000 BTU/hr = 3634 seconds

Some of it will have steamed out already.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[cdill21]

EdStainless, we have not tested this particular failure mode. What tends to happen first is the welds that hold the heaters in crack from added thermal stress when an air or steam bubble builds up. This results in leaking water.

 

[Compositepro]

I believe IRstuff means latent heat of vaporization. But that is not relevant here since, in case of the contactor failure the water will not be vaporizing but building pressure. It is usual that such devices have completely independent controls for temperature, pressure, and pressure relief. What is the ASME pressure rating of the tank and the pressure setting on the relief valve? It certainly does not sound right that you have 30,000 BTU/hr of heat and only 15,000 BTU/hr of relief capacity. However, the relief capacity depends on the pressure setting. It does sound like this device would not be legal in most jurisdictions, even though it may not actually explode.

 

[MJCronin]

Some questions:

1) Who fabricates the unit ? Does it carry any national certification, identification, model number or periodic testing certification ?

2) Do you have a pressure/temperature gauge somewhere on it ?... What pressure does it run at ?

3) Is there a UL/ASME/ASHRE or other certification on the heater and tank ?

MJCronin
Sr. Process Engineer

 

[cdill21]

The relief valve is set to 150 psi and the unit runs at about 135 psi and 250°F regularly. The tank has not gone through any rigorous ASME BPVC analysis. As far as I can tell, the maximum allowable pressure was calculated to be 700psi based on the hoop stress of the tank to yield and a SF of 3.0.

I oversimplified when I called it a small water heater: the machine uses a 2 HP pump to send water to and from a process, which dictates the pressure. The machine also uses cooling water from a chiller or city water line to control temperature.

 

[cdill21]

MJCronin, the unit was designed by engineers at my company that left less than satisfactory documentation. It is sold with and without UL and cUL certification, but it appears to only concern the electrical enclosure. We do not change any mechanical components to achieve UL certification. There is no ASME or ASHRAE certification anywhere.

 

[MJCronin]

I believe that this thread should be in the "Boiler and Pressure Vessel" Forum...

I also believe that there may be serious certification and safety issues here .... You may be in violation of State/Provincial laws or Statutes.

Are you located in the US or Canada ???

MJCronin
Sr. Process Engineer

 

[djs]

Is the pressure relief valve piped to vent water or steam?

 

[LittleInch]

Where's the pressure rise coming from?

Does the machine shut down with closed valves?

Or are you looking at all possibilities?

So its no incoming flow but heat still on. So long as there is a path out then there shouldn't be a pressure rise?

Thought about a bursting disc?

Where does the relief valve relieve to at the moment?

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

 

[cdill21]

These units are sold across North America, but are used primarily in the United States. They are manufactured in Pennsylvania.

The pressure relief valve is piped to vent water, not steam.

The expected failure mode is the cooling water valve stuck shut with the heater contactors welded closed. This leads to uncontrolled heating in the tank inside of the closed system. The increased pressure will come from the water heating up. The only drain is the pressure relief valve.

We had not thought about a bursting disc.

 

[LittleInch]

It might be water when it enters the relief valve but it will be steam when it comes out....



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

 

[Compositepro]

A relief valve rated in BTU/hr is designed to vent steam.

 

[cdill21]

Yes, that makes sense. I take back what I said about it being piped to vent water.

As I understand it, there is a CSA rating and an ASME rating for the valve, where the CSA rating will only ever account for a 15 psi pressure drop(10% of 150psi, which most water heaters are rated for). For a 1/2" valve, this is always 15,000 BTU/hr. ASME would allow a much higher pressure drop based on the design pressure of the system, though this is not listed on the valve, though I am not sure if this requires the pressure vessel to be stamped or not.

Would it be kosher to cross post this to the "Boiler and Pressure Vessel" forum?

 

[dgallup]

I think the way to get it moved is to red flag it yourself and ask the mods to move it.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MJCronin]

These people seem to make a competitive product to what you offer.

http://www.reimersinc.com/

Their minature electric steam boilers (AR Series) are designed, fabricated, inspected and tested in accordance with the requirements of the ASME Boiler and Pressure Vessel Code and the control systems meet ASME CSD‐1. Each boiler bears the National Board Stamp “M”.

There are two fuel safety cuttoffs, for high pressure and for low water level

http://www.reimersinc.com/_literature_183008/Model_AR_Steam_Boilers

In my opinion,I believe that your equipment should have similar qualifications and features

MJCronin
Sr. Process Engineer

 

[cdill21]

MJCronin, thank you for your recommendation. After some thorough diving through ASME BPVC, it suggests that there is no risk of a steam explosion (all water in the system will boil before the pressure vessel is at risk of cracking). The ultimate end to the uncontrolled heating failure mode is continuing to heat the steam in the system until the heating coils melt and quit drawing current. Then we are left with a smoking hot boiler that is filled with extremely hot steam. Ideally, the operator will pull the plug when the pump starts making horrendous noise when cavitation starts as the water on the low-pressure side starts to boil. Alternatively, I would hope they pull the plug when smoke starts billowing out from the seals, insulation, and paint burning.

Thank you everyone for your contributions. This truly is the one forum I can count on for expert advice.

 

[Benj107]

It is not clear for which "expert advice" you gave thanks.

Did anyone provide a reassuring answer to your subject query?

This expectation is disquieting: "when smoke starts billowing out from the seals, insulation, and paint burning."

This suggests that all relevant failure scenarios have not been considered: "The expected failure mode is ..."

Users of products can find innovative ways to create situations that were not anticipated by a designer.

Maybe someone closed a manual valve that prevents water from flowing to the unit?

What if someone walked away from the unit after closing a valve but didn't turn off the heater? When they re-enter the room to find "paint burning," they might decide the cure is to open the valve to cool the "smoking hot boiler that is filled with extremely hot steam."

It could be a recipe for a bad day.

If there is a low probability that someone from your company will be in the room if something goes south, a better question for the post might be "Is anyone at risk of a steam explosion?"

You might run that question past the company's product liability insurance carrier.

 

[chicopee]

One item that stood out in the original post and I noted one comment by Compositepro relating to it is that the relieving capacity of the safety valve should be not less than the heat input to the water heater. Also you should be careful that the discharge pipe on the outlet of the safety valve is not reduced in size or too long which can create excessive back pressures. Regarding the leaking weld(s),it is possible that make up cold water replenishing the heater during its operation impinges on hot surfaces of the heater and that is not a desirable water circulation.