SUPERHEAT STEAM MAXIMUM PRESSURE AND TEMPERATURE

[ENG-RAY]

hi,
I am working in a chemical plant which receives steam from a supplier. We receive super heat steam with temperature between 180 deg C to 250 deg C and pressure between 10 to 11 barg. There is a safety valve with set pressure of 11.4 barg at interface point and also some temperature indicator. No pressure alarm and control is available. Supplier has a temperature shutdown somewhere in their interface but the set point is 300deg C. Till now we always considered saturation temperature of steam at 11.4 barg which is 190 deg C for design of all equipment. Now the authority inspector said that it is not acceptable anymore because we do not have anything to limit temperature so we should consider 300 deg C. As you can imagine it will affect the whole plant.
Do you have any idea how we can solve this problem? is it realistic to consider 300 or even 250deg C?
we can add one alarm on steam line on interface but it is not possible to close any valve when it happens.
In which condition we will experience superheat steam in plant. All temperaure indicator at interface of equipment show no more that 190 deg C.

 

[LittleInch]

I don't know if its me but there seems to be an issue here between these statements, [my emphasis]

"We receive super heat steam with temperature between 180 deg C to 250 deg C"
and
"is 190 deg C for design of all equipment."

SO you already seem to know that you can receive superheated steam, which by its very definition is a higher temperature than the saturated steam temperature, at a temp up to 250C, but design only to 190??

The setting of the safety valve at 11.4 appears to be very close to your max delivery pressure of 11 bar - too close for reliable operation IMHO.

In any system receiving fluids you need to know what the upstream temperature conditions are and what systems are in place to limit pressure and temperature. In this case it doesn't sound like you really know - "temperature shutdown somewhere in their interface" - which is never a good start.

So I think the authority inspector is correct - you need to design your system for somewhere between 250 to 300C. Design ot 190 is only valid if the upstream supplier guarantees it and demonstrates that the temperature will not exceed that due to alarms, trips and shutdowns of an adequate nature.

If you can't close any valve on an alarm or trip signal then again this is not normally what happens. The receiving plant / process equipment MUST be in control of it's inputs to protect itself. Or it MUST be designed to handle whatever the upstream system can throw at it.

It's one or the other.

Your last sentence makes no sense when compared to the first sentence. If you are only getting saturated steam (190C) then why do you say you get superheated steam at up to 250C??

10 to 11 bar steam is a saturated temp of 184 to 188C.

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[TiCl4]

Let me get this straight. You already receive steam up to 250 C from the supplier, but you have 190 C as the design condition for your vessels? You are already exceeding the maximum design metal temp of your vessels! Why would you have only considered saturated steam conditions if you are receiving superheated steam??? I think the inspector is correct in allowing for maximum incoming superheat temperature at 300 C, but you are already in deep trouble with your incoming steam regularly exceeding your design limits.

Your cheapest option, IMO, would be to install a de-superheater unit to ensure the steam is saturated and meets all your vessels' original design conditions, which apparently only considered saturated steam at 11.4 barg. If you ensure you have saturated steam only, then a single PSV on the header can ensure the header conditions never exceed your vessel design conditions.

Is saturated steam acceptable for your process?

 

[ENG-RAY]

Thanks for the fast reply. Unfortunately the statement is correct. We receive Superheat steam at our interface and the pipe is designed for temperatue up to 250 deg C. The plant is big and till the pipe reaches to each single equipment , the steam is saturated and not Superheat anymore ( this can be confirmed by temperature indicator which we have on steam pipe close to our equipment). Therefore we are not actually exceeding the maximum design metal temp of our equipment but we can not say that it is impossible.
The plant is old and I guess in the past we did not have this high temperature at interface. Local regulation also allows to consider saturation temperature for hazardous area classification and hot surface temperature.
if we use Desuperheater ( considering the fact that actulaly without it we have saturated steam when steam reaches to Equipment), I guess we will have more condensation on pipe and steam consumption will drastically increase. Am I right?

Is it normal to have that much temperature loss ( 250 deg C to 190 deg C)? the steam pipe are isulated.

 

[TiCl4]

It’s good to hear that you have enough energy loss to de-superheat the steam by the time it reaches process vessels. Without a lot more details, it’s impossible to know if that level of heat loss is normal or not.

I still think you should install some type of control - you are relying on natural losses that may change in the future. Maybe your current insulation is compromised and will be replaced in the future. That could lead to high temperatures.

If you want to keep your current ratings, perhaps you could perform a heat loss analysis on your main header to enshrine it as an “engineered” system whose purpose it is is de-superheating. That way, any changes to the header in the future should reference those calculations in the MoC to avoid future issues. You would need to consider the input temp as 300 C, in this case. Based on this analysis (300 C incoming temp on the hottest day of the year), you may be able to justify saying supply temperature will not exceed 190 C.

The safe, but more expensive alternative would be to install a de-superheater just upstream of the first branch. If the steam is already saturated by that point, the de-superheater won’t do very much most of the time, but would kick on in the case of a process upset at the supplier that sends higher steam temperature than normal to you.

You are in an awkward spot, no doubt about it.

 

[LittleInch]

I agree with TIcL4.

You will need to work at showing that your rather large temp loss cannot be exceeded in any circumstances at the equipment delivery point.

I have no idea if that is possible or not - only you can see your system.

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

 

[hacksaw]

What is the steam supplier's piping rated for (which code designation) and size?

You also meed to know the pressure setting of their pressure safety valves. You'll be required to design to their standards.

 

[georgeverghese]

Another solution may be :
a) Request to lower the temp shutdown - trip at the supplier to 250degC to match your main incoming steam supply header design T
b) Install desuperheater at each consumer within your plant to enable sat steam at < 190degC and also install temp shutdown at each supplier, downstream of local desuperheater, set at 190degC.

It is difficult to see how superheated steam temp can drop from 250degC to 185degC or less inside your plant unless the insulation on the supply headers is in tatters.

 

[TBP]

This situation has the vibe of mixing gauge pressures and nameplate values in the field with steam table values in absolute pressures - along with the corresponding temperatures.

 

[pierreick]

Hi,
The OP should consider the link attached to assess the quality of steam needed for his process:

https://www.tlv.com/global/TI/steam-theory/types-of-steam.html

In term of heat transfer , saturated steam is better than Superheated steam .
Note : With a de-super heater in service, saturated steam is generated !

Pierre

 

[TBP]

In plants like this, it's very common for there to be a pressure reducing valve on the customer's end of the steam supply line. Is there a PRV station just before the safety valve that's set for 11.4 bar? If there is, what is the steam temp & pressure upstream of the PRV? What is the PRV setpoint?

11 bar is 150 PSI and 11.4 bar is 165 PSI. Operationally, that should work just fine. I'm betting that this plant is full of 150# valves, flanges & fittings because folks were looking at the operating conditions and not the where the safety valves needed to be set. I've seen this happen a number of times.

 

[LittleInch]

Errr 11 bar is 160 psi not 150....

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Also: If you get a response it's polite to respond to it.

 

[TBP]

So it is, LI. Thanks

I don't "think" in metric units - I have to convert everything.

 

[LittleInch]

That's OK, we're all used to certain units. Does this impact your comment? 5 psi in 160 seems like a very low margin to me.

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