Valves on steam lines are commonly encountered and you should know how they work

Question

Valves on steam lines are commonly encountered and you should know how they work. For most valves, the change in velocity of the fluid flow is negligible. Apply this principle to solve the following problems: A pressure transducer on a high-pressure steam line reads P_1 = 80 bar (absolute), but temperature measurement is unavailable inside the pipe. A small quantity of steam is bled out through a valve to atmospheric pressure at P_2 = 1 bar. A thermocouple placed in the bleed steam reads (T_2) = 400 degree C. What is the temperature (T_1) inside the high-pressure duct? Steam traps are common process devices used on the lowest points of steam lines to remove condensate. By using a steam trap, a chemical process can be supplied with so called dry steam, i.e., steam free of condensate. As condensate forms due to heat losses in the supply piping, the liquid runs downward to the trap. As liquid accumulates in the steam trap, it causes a float mechanism to move. The float mechanism is attached to a valve, and when the float reaches a control level, the valve opens to release accumulated liquid, then closes automatically as the float returns to the control level. Most steam traps are constructed in such a way that the inlet of the steam trap valve is always covered with saturated liquid when opened or closed. Consider such a steam trap on a P_1 = 7 bar (absolute) line that vents to P_2 = 1 bar (absolute). What is the quality of the stream (q_2) that exits the steam trap at P_2 = 1 bar?

Explanation / Answer

a)

P1= 80 bar

T1=?

P2= 1bar

t2= 400degC

for P2,1bar and T2=400degC

and Tsaturated is greater than T2 form table

hence we assume that feed is superheated steam and we consult the superheated steam table

hence  H2= 3.27*106

since velocity is constant

H2= H1

there for P1= 80 bar and H1=3.27*106, from the table we find the corresponding temperature

hence the T1= 449.72 deg C

b) P1 = 7 bar

P2 = 1 bar

since valve is always covered with saturated liquid, therefore we take temperature T1 as saturated temperature from saturation table of water

hence T1= 285.83deg C

now from saturated table of superheated steam we find H1 for P1=7bar and t1= 285.83 deg C

hence H1 = 3028kJ/kg

for energy rate balance, H1=H2

now for P2 = 1bar we find

hf1= specific enthalapy of superheated water = 417.438 kJ/kg

hfg= Latent heat of steam = 2257.51 kJ/kg

therefore q2=quality of steam =( h1-hf1)/hfg =(3028-417.436)/ 2257.51 = 1.15

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