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Amount Of Flash Steam / Appropriate Rupture Disc


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#1 rinke

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Posted 01 September 2014 - 03:27 AM

Hello, I’ve got a (in my opinion) difficult relief case in which I hopefully can get some assistance.

The case is as follow:

A storage vessel of 4.5m3 contains water 2.3m3 water at 1.5 barg and 127.6°C. The water is pumped in a loop and is fed back again to the vessel. Heat losses are compensated with an external heater in the return of the loop.

The vessel is equipped with a rupture disc (43mm opening diameter) which vents to an atmospheric safe location via a 4mtr long 44.3mm internal diameter line. In case the heater will heat to much the rupture disc breaks at 3.0 barg (144°C) and the pump stops pumping and heater stops heating. So, no new heat input to the vessel is provided. Theoretically the rupture disc must be able to handle the amount of flash steam being generated… Question is, how can I calculate the amount of flash steam being produced per second (kg/s) and if the rupture disc is appropriate for the system? In my opinion the dynamic system needs simulation to give an answer or can I make some assumption and do a rough hand calculation and prove whether or not the disc is correct sized?

Thanks in advance for tips and hints!!!

 



#2 PingPong

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Posted 01 September 2014 - 05:47 AM

In case the heater heats too much the water temperature in the tank will gradually rise, along with the pressure.

 

You need to calculate the maximum possible duty of the external heater when the circulating water has reached 144 oC (and pressure has risen to 3 barg), divide that by the heat of vaporization of water at 144 oC and that gives you the flash steam flowrate.



#3 rinke

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Posted 01 September 2014 - 07:21 AM

Thanks for your reply PingPong. The water is needed to sterilize the loop at 1.5 barg. So, the water is held in the storage vessel at boilingpoint. If the temperature of the water rises, also pressure will rise. However the limit is 3.0 barg. In that case the disc will rupture and the water will boil out. Because the circulation pump stops and also the heater will stop heating there is zero input to the vessel. In that case I don't see the relation between heater duty and  initial flash steam flow produced. max pressure of black steam at heater is 3.5 barg, flow of water is 3.5m3/hr..

 

The way I was thinking is that I can calculate the total amount of flash steam being produced out of 2.3 m3 water @3barg and 144C but I do not know in which time it will be flashed...



#4 PingPong

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Posted 01 September 2014 - 08:14 AM

If all instrumentation and safeguarding trips would always be guaranteed to work, we would never need any RD's or PSV's.

 

The RD + piping has to be big enough to relief the maximum possible vapor generation inside the system, which is determined by the maximum possible heater duty when the water is at 144 oC. So you need to dig up the heater datasheet.

 

max pressure of black steam at heater is 3.5 barg
What does that mean?

#5 rinke

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Posted 02 September 2014 - 02:41 AM

sorry.. working in pharmaceutical bussiness ;)  black steam = house / plant steam.

But I still don't get it (maybe thinking to difficult :( ). Why should the heater have influence if the system will stop running and heating. Only flash steam generated from the water bulk is expected isn't it in that case? In case the pump will go on, I can imagine that 3.5 m3/hr of water at 144°C will directly evaporate into flash steam.

 

About a datasheet, I don't have one available. Only extra data from the heater I have is:

 

max working pressure (Shell / tube) 10 barg

Max working temp (shell / tube) 184°C

Volume tubes (water side) 1.01 liter

Volume shell (steam side) 3.27 liter

 

steam fed to the heater is equipped with a PSV at 3.5 barg


Edited by rinke, 02 September 2014 - 03:12 AM.


#6 PingPong

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Posted 02 September 2014 - 03:37 AM

Why should the heater have influence if the system will stop running and heating.

.....

I can imagine that 3.5 m3/hr of water at 144°C will directly evaporate into flash steam.

As I wrote before: if all instrumentation and safeguarding is guaranteed to work an RD is not required at all.

Water at 144 oC will not generate flash steam at 3 barg.

But RD's and PSV's are installed to make sure that even in the worst case scenario the design pressure of the protected equipment is not exceeded.

 

In general: a system containing a substance that can vaporise, and receiving heat input, needs a PSV or RD with sufficient capacity to relieve the maximum possible vapor production due to that heat input.

It does not matter whether that system is your water tank, or a distillation column with a reboiler, or whether the heat is a result from a fire.

 

Without more heater data you cannot calculate the heat input.

 

There are a few things that I do not understand here:

1) Why does this water tank have an RD instead of a PSV ? Are there any other relief cases applicable for this RD except the one due to heat input?

2) Why do you need to calculate the relief load for an existing tank with an existing RD ? Has the heater duty increased in some way?

 

Try to find the RD datasheet and see what relief case(s) and flowrate(s) was specified there to size the RD.


Edited by PingPong, 02 September 2014 - 03:44 AM.


#7 rinke

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Posted 02 September 2014 - 05:56 AM

If I evaluate the system a rupture disc is indeed not needed looking at all safeguardings etc. However I just wanted to know if the RD can handle the theoretically generated steam flow in case of rupture... To answer your questions:

1) RD instead of PSV because of sanitary / hygienic design (pharma world / WFI storage). Only heat case could be a fire of the building...

2) During delivery of the system no RD calculation was provided. This has to be done conform the validation document written. However it might be better to write down a statement in which is stated that the rupture disc will never be used except a fire...

Rupture disc has just been chosen, no scenario was found...



#8 PingPong

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Posted 02 September 2014 - 09:11 AM

However I just wanted to know if the RD can handle the theoretically generated steam flow in case of rupture...
Now you are talking about tube rupture?

In the petroleum and petrochemical industry that I work in, tube rupture need not be considered to size the RD or PSV if the design pressure of the exchanger low pressure side is at least 70 % of that of the high pressure side. I don't know what is normal practice in the pharmaceutical industry. That does not mean however that the RD may not fail if tube rupture does happen, causing a relief. I hope you understand the difference.

In any case it is possible to do a calculation of the maximum possible steam flow through a ruptured tube if you know the inside diameter of the exchanger tubes.

 

During delivery of the system no RD calculation was provided. This has to be done conform the validation document written.
That is the world upside down.

So the system vendor provides a 43 mm RD (with a 44 mm ID pipe) and then expects somebody else to produce a calculation that validates that size?



#9 rinke

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Posted 03 September 2014 - 12:29 AM

no no no.. Maybe a writing mistake because I'm not english / american ;) with rupture I meant breaking of the RD.... Tube ruptures I have calculated in the past, so that shouldn't be a problem thanks :) about the calculation, I phoned the RD vendor and he will send me the calculations this week ;) However, I also wrote a note myself that because of all safeguarding the RD will not break except for a fire case...






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