5 replies to this topic

[J_Leo]

The continuous blowdown rate from a heat recovery steam generator is 5%. The same pipe size is used for both continuous and intermittent blowdowns. The angle valve size is also the same for both. The steam pressure is blown down from about 21 kg/cm²g to near 0 kg/cm²g.

Since the blowdown water is saturated, choked flow will occur with such a big pressure drop. What is the maximum flow rate of the intermittent blowdown? How do I calculate it?

I tried the equation found in the Emerson Control Valve Handbook,

qmax= N1 *FL*Cv*SQRT((P1-FF*Pv)/Gf)

but what Cv do I use? --Cv from valve data, or the calculated Cv under choked flow?

[Root]

Use Valve data CV because you are calculating for exiting system not for new system.

Toor

[kkala]

Following on the subject may be helpful.
1. Some insight can be on the thread {url]http://www.cheresources.com/invision/topic/12649-flow-through-restriction-orifice/page__p__48477__hl__%2Btwo-phase+%2Bflow__fromsearch__1#entry48477[/url]. I have not seen DIERS method applied for valves, probably due to its complexity.
2. I think there is actually no Cv for choked flow. Upstream pressure determines the flow rate through the valve (just as in a PSV or orifice for choked flow) and downstream pressure is determined by the downstream piping ΔP (e.g. going upwards from pipe end to the valve).
Irrespectively of above, Carl R. Branan's "Pocket Guide to Chemical Engineering" (Gulf Publishing Co, 1999), Chapter of "Control valve design", Liquid flow (p. 16 and on) presents calculation of max allowable ΔP for sizing purposes that may be useful.
3. Flashing makes the issue complicated, needing trial & error. In a manual calculation I would neglect liquid downstream of valve at first trial (vapor has much higher volume), considering only flashing vapor of increasing quantity (as the pressure goes lower and lower).
A simulator can be of real help in this case (then liquid phase would not be neglected).
4. In case that blow down valve is an on / off valve (activated by level, I remember one such boiler case here) much precision is not needed.

[J_Leo]

Thank you. I am trying to figure out if it is right to use Control Valve Cv for max mass flow rate calculation.

[J_Leo]

kkala,
Thank you very much for your kind input. Infact, my purpuse is to design the downstream equipment(for example, the Blowdown Drum)and try to figure out what might be the intermittent flow rate compared to the continuous blowdown. Currently, I use the same flow rate for intermittent flow as the continuous blowdown, because I think the maximum flow rate through the valve should not be much bigger compared to the continuous blowdwon, even when the intermittent blowdown angle valve is fully open.

Edited by lew, 05 June 2011 - 09:05 PM.

[kkala]

1. As an example, blow down sizes and valves could be as follows for a 60 ton/h steam boiler (41 Barg, 400 oC).
Continuous blow down (max 10%) line 1” upstream control valve, 3” downstream. of it. Cv of the control valve (size 1”) about 4.5 (about 3.0 required for max controllable flow, 0.5 for minimum).
Discontinuous blow down line 4”, only with manual valves, without control valve. It seems that much higher flow is instantaneously needed by the discontinuous blow down to flush the boiler.
Continuous blow down comes from steam drum, discontinuous from steam and mud drum. I would expect blow downs to come from mud drum.
2. Concerning valve sizing under flashing, C. R. Branan (Rules of thumb for chemical Engineers, Gulf 2002) specifies max ΔP effective in producing flow (max allowable ΔP for sizing purposes). I understand that actual Cv is used. Higher ΔP would not produce more flow due to flashing.
This might be worth trying, understood as a short method of choked flow through valve (like an orifice) and downstream pressure separately (as said in previous post).

Edited by kkala, 08 June 2011 - 03:44 PM.