blowoff tank design / operating pressure 1

[davenaude]

Hi all. I am designing a blowoff vessel for a 90 bar recovery boiler in accordance with ASME 8.1. The blowoff is not subjected to any superheated steam. I am trying to source an accurate method to calculate the pressure experienced in the vessel during blow down. The blowoff vessel is vented to atmosphere and an operating and isolating valve pair in series is situated at the take-off from the steam drum for the continuous blow down as well as at the mud drum for bottom / main blowdown.

any suggestions would be appreciated.

 

[davefitz]

You will need to calculate the max flowrate of saturated liquid thru the blow down lines to the tank, based on the drums being at max drum operating pressure. This is a trick calculation ,as there may be 2 phase flashing flow in the blowdown line and valves.

After this blowdown flow is determined , then you calculate how much steam is developed in the flash tank by an isenthalpic seration of the saturated liquid originally at max drum pressure into a mixture of sat liquid at 15 psig and sat steam at 15 psig ( first estimate).

The pressure in the tank is equal to atmospheric pressure plus the pressure drop thru the exhaust vent. If the vent is undcersized, you could get chiked flow thru the vent pipe, which would thenimply the vent calculation would match teh procedure used to size saferty valve exhasut pipes ( see ASME sect VIII div 1 appendix). If not choked , then normal pressure drop calcs are used.

 

[sailoday28]

I agree with davefitz (Mechanical)except that energy balance for your well insulated tank and inflow lines should use stagnation enthalpy inplace of static enthalpy.

 

[sailoday28]

Using enthalpy of sat liq and vapor in flash tank, will approximate their stagnation enthalpy. Using enthalpy of blowdown from boiler will approximate its stagnation enthalpy.

 

[chicopee]

Is this blowdown tank for surface blowdown(that is at water level), bottom blowdown or both?

 

[davenaude]

Thanks all for your feedback. I will be using stagnation enthalpy. Continuous blowdown is at or just below the water surface in the steam drum whilst the bottom blowdown is at the lowest point of the mud drum.

regards davenaude