Steam Control Valve Specification Problem

Pavan Kumar · Aug 8, 2022

Hi All,

I need guidance to know if the way I am calculating the pressure upstream and downstream of a steam control valve for three different flow rates for its specification is correct or not.

The system sketch is attached.

I have to specify a Steam Control valve that controls steam flow to a Butane Vaporizer, through a steam bundle, that vaporizes liquid butane to vapor butane. The butane vaporizer has a Steam bundle immersed in liquid butane supplied from the bottom nozzle. The Steam control valve supplies steam to the Steam bundle at the desired opening which is controlled by the butane vaporizer pressure. The desired vaporizer pressure is 55 psig. The min, normal and max liquid butane flow rates are 10, 36 and 40 gpm respectively.

Taking the normal flow rate case, I determined that the required steam flow rate is 2659.38 lb/hr of superheated steam at 50 psig. As you can see in the sketch the steam is supplied to the control valve from the 265 psig Saturated (HP) steam header through a pressure let down regulator that is set to reduce the steam pressure from 265 to 70 psig pressure. Then there is a 80 ft long 3" Sch 40 [pipe between the let down regulator and the steam control valve. There is 2 feet of 3" sch 40 pipe between the downstream of the control valve and the steam bundle.

A) To calculate the steam pressure and temperature at the control valve inlet, I calculated the following :

1. I calculated the Steam super heat temperature for isenthalpic expansion from 265 psig to 70 psig to be 175.79 Deg C

2. I calculated the heat loss from the 3" sch 40 pipe with 2" mineral wool insulation and it is 7472.6 Btu/hr after considering the resistance of ambient air at 21.1 Deg C ( 70 Deg F).

3. I calculated the steam pressure prop using isothermal flow equation and calculated it as
1.5 psi. This means the steam pressure at the Control valve inlet is 70-1.5 =68.5 psig.

4. The specific enthalpy of Superheated steam at 70 psig is 1202.49 Btu/lb. Substracting the heat loss of 7472.6 from the heat content of steam at 70 psig ( 1202.49 Btu/lb)flowing at 2659.38 lb/hr, I get the specific enthalpy at the control valve inlet as 1199.68 Btu/lb

Specific enthalpy at Control valve inlet= Heat content of 70 psig superheated steam - Heat loss
= (2659.38*1202.49-7472.6)/ 23659.38 = 1199.68 Btu/lb

Knowing the steam pressure = 68.5 psig and specific enthalpy = 1199.68 Btu/lb at control valve inlet I get the steam condition to be superheated with superheat temperature to be 172.68 Deg C.

So at the control valve inlet:

Flow Rate = 2659.38 lb/hr
Pressure = 68.5 psig
Temperature = 172.68 Deg C
Specific Enthalpy = 1199.68 Btu/lb
Condition = Superheated.

B)To Calculate the Steam pressure and temperature at control valve outlet:

1. I fixed the pressure at the steam bundle as 50 psig and then back calculated the pressure drop for the flow of 26549.38 lb/hr to be 0.78 psi. So the pressure at the control valve outlet is 50.78 psig. With the steam expansion across the control valve being isenthalpic, the specific enthalpy at the control valve outlet is also 1199.68 Btu/lb

2. Calculated the heat loss in 3 feet of 3" sch 40 outlet pipe for 2659.38 lb/hr as 267.9 Btu/hr.

3. This gives the condition of steam at the bundle inlet to be 1199.579 Btu/lb. At 50 psig and 1199.579 Btu/lb the steam is superheated with temp = 168.547 Deg C.

At the bundle inlet the condition is

Pressure = 50 psig
Temp = 168.547 Deg C

Using this information for the required heat load of 2.481 MM Btu/hr required to maintain 55 psig Vaporizer pressure at 40 gpm liquid butane ( at 0 Deg C), I recalculate the required steam flow rate. Then after a couple of iterations I arrive at the following conditions at teh control valve inlet and outlet.

Case1: Butane Flow rate = 40 gpm

At Control Valve Inlet:
1. Steam flow rate = 2659.38 lb/hr.
2. Pressure = 68.5 psig
3. Temperature = 172.628 Deg C
4. Specific Enthalpy = 1199.68 Btu/lb
5. Condition = Superheated

At Control Valve Outlet :
1. Steam flow rate = 2659.38 lb/hr.
2. Pressure = 50.78 psig
3. Temperature = 168.547 Deg C
4. Specific Enthalpy = 1199.68 Btu/lb
5. Condition = Superheated

If this way of calculation is correct I will run the calcs for Cases 2 and 3 also and send the specification to the vendor.

Is it advisable to use polytropic process to calculate the gas pressure as opposed to isothermal / adiabatic and exclude all the pipe heat loss calculations?. There is a paper by William M. Kirkland ( attached in next message) which shows how mass flow can be calculated using polytropic approximation of compressible flow.

Your suggestions will be very helpful to me.

Thanks and Regards,
Pavan Kumar

Pavan Kumar · Aug 8, 2022

Paper "A Polytropic Approximation of Compressible Flow in Pipes with Friction" by William Kirkland attached.

georgeverghese · Aug 9, 2022

You say liquid butane boils at 0degC at 55psig ? Saturated butane table in Perry tells me boiling temp is 322degK at 55psig = 49degC.

Pavan Kumar · Aug 9, 2022

Hi georgeverghese,

Liquid butane is fed into the vaporizer vessel at 0 Deg C. It vaporizes at 46 Deg C at 55 psig.

Thanks and Regards,
Pavan Kumar

MortenA · Aug 9, 2022

Funny mix of units though. Its going to give any vendor a headace and increases the risk of errors in unit conversion.

3 digist on you temperature is overdoing it (i would go for no digits) - same for most of the other values.

But your method seems fine. I would use a steam table for the calculations because steam is so well known.

You mention the gas pressure (not steam i assume) but i dont see what this has to do with the steam side? Your heat input for evaporating is of course related to your butane evaporation rate but only via the latent heat i would assume and your butane consumption?

Best regards, Morten

--- Best regards, Morten Andersen

pierreick · Aug 9, 2022

Hi,
I agree with Morten , you should be consistent with your units system . To me better to use absolute pressure instead of gauge
to prevent confusion , reading tables or performing calculations..
I've added a calculator for steam properties if needed.
Regarding the method, it's fine .

BTW : thanks for sharing your document.

Good luck
Pierre

Pavan Kumar · Aug 9, 2022

Hi MortenA,

Please see my responses below.

MortenA said:
Funny mix of units though. Its going to give any vendor a headace and increases the risk of errors in unit conversion. Yeah I will convert them to all English units. It only the temperature thsat is n Metric units.

3 digist on you temperature is overdoing it (i would go for no digits) - same for most of the other values. Yeah point taken

But your method seems fine. As you have seen I have calculated heat loss and pressure drop separately. Would using the polytropic constant ( based on Kirkland's paper ) help calculate the pressure drop more accurately after taking the heat loss to ambient into consideration?.

I would use a steam table for the calculations because steam is so well known.
I did use Online steam table data from the link copied below.

https://www.spiraxsarco.com/resourc...m-tables/superheated-steam-region#article-top
for my calculations. Going forward I will use the values from the spreadsheet sent by pierreick below.

You mention the gas pressure (not steam i assume) but i dont see what this has to do with the steam side? Your heat input for evaporating is of course related to your butane evaporation rate but only via the latent heat i would assume and your butane consumption?

Yes the butane pressure to be maintained is 55 psig by vaporizing 40 US gpm liquid butane entering at 0 Deg C. So the required steam flow rate is calculated to cater to the heat load from sensible heating of liquid butane from 0 Deg C to 49 Deg C( saturation temperature of n-butane at 55 psig pressure) and latent heat of vaporization of butane at 49 Deg C . I fixed the steam side pressure at 50 psig and based on the calculations I found that the steam will be superheated and 2659.38 lb/hr is required.

Thanks and Regards,
Pavan Kumar

Pavan Kumar · Aug 9, 2022

Hi pierreick,

Thank you so much for your guidance and the Steam Table spreadsheet. I will use it for this calculation to help automate my calculations. Thanks a lot as for your help.

Thanks and Regards,
Pavan Kumar

Pavan Kumar · Aug 9, 2022

Hi pierreick,

The spreadsheet you gave me does not seem to work ( or may be I am missing something). The moment I enter a value for example 50 Deg C for Saturation properties given temperature, then all the properties are displayed as "#NAME?". Is there a way to deal with this?.

Thanks and Regards,
Pavan Kumar

pierreick · Aug 9, 2022

Hi,
I don't see any problem with the spreadsheet , you have to reply OK when you open it then go to properties and fill the data required into the blue shaded location .
If it did not work for you you can use this app from the net with many more calculators :

https://www4.eere.energy.gov/manufacturing/tech_deployment/amo_steam_tool/

Pierre

georgeverghese · Aug 9, 2022

a) If liquid butane is fed at 0degC to this steam bundle, then when steam supply is temporarily stopped to this unit during normal operation (say when butane vapor demand is low or near zero), steam will freeze in this bundle at 0degC. Restarting this vaporiser wont be possible after temporary disruption of steam supply. Can you raise the liquid butane feed temp to say 5-10degC to prevent steam from freezing?
b)You have allocated a dp of approx 20psi to this control valve. Where does steam condensate go to after the trap? What is the max backpressure in the return condensate line at this location after the trap? Steam condensate return lines are notorious for slugging and fluctuating backpressures.

Pavan Kumar · Aug 10, 2022

Hi pierreick,

Thanks. I unblocked the Macros and it is working well now. Do you have one for English units?. It would be very helpful if you can share. The calculators in the link provided by you are very useful. I can use the Steam Turbine one right away. Thanks a lot once again.

Thanks and Regards,
Pavan Kumar

Pavan Kumar · Aug 10, 2022

Hi georgeverghese,

1. There is a never a ,ow case for butane as there is flow as longs as the plant is running. When the reactors are shutdown the butane flow goes to zero. So the chances of Steam freezing is quite rare.

2. Liquid butane is supplied to the vaporizer from liquified butane storage bullets through a positive displacement pump. These bullets are kept under pressure, but the temperature of liquid butane varies with the ambient conditions. In the winter conditions the temperature drops even to sub zero temperatures here. I was calculating the heat load for a reasonable worst case. I actually should be using -10 Deg C to be conservative. The liquid butane temperature goes to 30 Deg C during summer time.

3. The condensate after the trap goes to a condensate flash tank which collects Condensate from other sources and returns LP condensate back to our Deaerator. There is some 20 feet of 1" pipe between the trap and condensate flash tank. Yes there could be flashing in the line and back pressure could well be present. How does this effect my heat transfer rate to liquid butane and the required steam flow rate?. Agreed the line downstream of the trap has to be sized to minimize this back pressure.

Thanks and Regards,
Pavan Kumar

georgeverghese · Aug 10, 2022

1) and 2) So it looks like in summer you dont have a problem when steam is left trapped in the HX when butane demand is low. But if the reactors are temporarily stopped during normal operation in winter, the vaporiser bulk pressure and temperature may drop < 0degC.
3) If there is a local LP condensate flash drum, and this condensate line slopes all the way down to the flash drum, then this wont be not be a problem.

FacEngrPE · Sep 3, 2022

In this application it is essential to ensure that the bundle will fully drain under all possible operating conditions. Including the off design startup case when steam is condensed so fast that the bundle partial pressure of steam is less than atmospheric pressure.

The design details needed to ensure condensate drainage include (but are not limited to) vacuum breaking, sizing of the steam trap, arrangement of the downstream condensate drainage system to minimize back pressure.

Pavan Kumar · Sep 7, 2022

Hi FacEngrPE, georgeverghese,

The pressure in the bundle is the incoming steam pressure. In our case it is 50 psig which will push the condensate out through the steam trap as long as the back pressure is less than 50 psig. The condensate line after the trap slopes to the Condensate flash tank. The line is 20 feet long and 1" Sch 40. Is there an technical reference that I can use to calculate the back pressure on the trap for two phase flashing flow in the condensate line downstream of the steam trap?.

Also it has been reported that the steam condensation inside the steam trap is quote rapid even during normal operation. We came to know this when we smelled Butane out of the condensate flash tank owing to the pin hole in the steam bundle. The butane entered the steam bundle from the shell side through the pin hole and ended up going to the condensate flash tank. The reason for this is the rapid condensation of steam that which is creating vacuum in the bundle. How can this be solved?. Will raising the steam pressure in the bundle greater than the vaporizer pressure of 55 psig ( see sketch attached ) solve this problem?.

Thanks and Regards,
Pavan Kumar.

georgeverghese · Sep 8, 2022

There are approximate methods for calculating dp in 2phase flashing flow in Perry Chem Engg Handbook - see the chapter on Fluid flow.
The pinhole in the HX bundle has to be plugged to prevent ingress of butane.
There are more than one process safety concerns with this setup, some of them not minor. This design will not pass a HAZOP, if you ask me.
At the very least, (a) the free draining HX bundle has to be made out of some corrosion resistant material (b) a baffle plate in the HX may help to enable a thermosyphon recirculation current that may minimise the likelihood of steam freezing during operation. But I dont see any way of stopping steam from freezing up in the tubes during shutdown.

Ideally, this butane should be vaporised with some hot oil (with operating range gong down to -10degC)at this HX. You could heat up the hot oil with LP steam.
But not LP steam directly at this butane vaporiser. For example, a hot oil such as Therminol 55 can be used down to -18degC.

Pavan Kumar · Sep 8, 2022

Hi georgeverghese,

I checked Perry's handbook 6th edition for Multiphase flow. On Page 6-28 ( 611 /2274 on the PDF) methodology has been proposed for calculation of pressure drop for two phase flow using the Homogenous equilibrium method but there is no clear cut formulae to calculate. This is similar to the paper's by JC Leung which I read earlier but was not clear to me. I want to know what the industry uses for two phase flashing flow pressure drop calculation. Any guidance will be very helpful to me. Per API 521 7th Edition 5.5.10 there is a procedure is given which I tried but could not get consistent results. Please let me know if you have tried it.

Yes we got the pinhole in the HX bundle repaired. We replaced the bundle with a new one while the pinhole was plugged.

georgeverghese said:
t the very least,

(a) the free draining HX bundle has to be made out of some corrosion resistant material : Yes we use SS316 as the bundle material for steam which I think is good enough.

(b) a baffle plate in the HX may help to enable a thermosyphon recirculation current that may minimise the likelihood of steam freezing during operation. But I dont see any way of stopping steam from freezing up in the tubes during shutdown. The steam bundle is immersed in the Butane vaporizer vessel. How will the baffles work and how would they be installed to have thermosiphon circulation?. I enquired with my supervisor who is also the Plant Manager. He said that steam will condense and cool but will be some 10 Deg C above freezing after it transfers heat to Liquid butane at 0 Deg C. He says he was never seen this happen. We have excess steam in our plant and would use it directly instead of using Hot Oil when it is not necessary.

Ideally, this butane should be vaporised with some hot oil (with operating range gong down to -10degC)at this HX. You could heat up the hot oil with LP steam.

But not LP steam directly at this butane vaporiser. For example, a hot oil such as Therminol 55 can be used down to -18degC.

Thanks and Regards,
Pavan Kumar

georgeverghese · Sep 9, 2022

For flashing flow, break up the pipe into 3-4segments, use an initial set of trial values for mean pressure for each segment and use the Lockhart- Martinelli procedure on pages 6-26,6-27. Change trial values to suit till they converge.
You can figure some way of making liquid butane at -10degC to 0degC to mix with the bulk liquid (so it gets prewarmed above 0degC) before it contacts the HX bundle.
During shutdown, as long as kettle vapor space pressure is kept high, you can avoid steam condensate from freezing up inside the heating bundle.
Ok, you've got a SS heating bundle, which is good, but not bulletproof. If you have a tube rupture during shutdown and butane rushes into the flash tank, condensate may freeze in the flash tank, block the vapor exit resulting in overpressure. Also, if C4 does not freeze, you've got a large flammable vapor cloud of C4 around the flash tank vent.

pierreick · Sep 9, 2022

Hi,
take a look at this spreadsheet.
Pierre

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