4. Steady state open system throttle valve (20 marks) Valves on steam lines are

Question

4. Steady state open system throttle valve (20 marks) 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.) A pressure transducer on a high-pressure steam line reads P1 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 P21 bar. A thermocouple placed in the bleed steam reads (T2)-400°C what is the temperature (T) inside the high- pressure duct? (b.) 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 Pressure transducer Pressure transducer P1 80 bar P High pressure stean line High pressure steam line Throttle valve Sat. vap Control valve Level transducer L Sat. T2 400°C P,-1 bar liq. Steam trap P 7 bar Pg 1 bar valve is always covered with saturated liquid when opened or closed. Consider such a steam trap on a P1 = 7 bar (absolute) line that vents to P2-1 bar (absolute), what is the quality of the stream (Q2) that exits the steam trap at P2 bar?

Explanation / Answer

For the first part, the main concept is that the enthalpy of the steam will remain same.

So we see the enthalpy of steam at 1 bar and 400 deg C.And then we search for the temp at 80 bar at which the enthalpy is same as previous.

Enthalpy = 3278.5 kJ/kg

At 80 bar pressure, from steam table, we see that at 450deg C, the enthalpy is 3273 kJ/kg which is very close tothe value we are looking for. From interpolation, the temp. is approx equal to 452 deg C.

(b) One of the things to understand here is that the steam is going to be saturated. This is because the steam is in contact with the liquid water.

So the calculation required is that when saturated water at 7 bar abs pressure flashes to 1 bar pressure, what is the quality of steam formed.

At 7 bar pressure, Enthalpy of sat water = 697.476 kJ/kg

At 1 bar abs pressure, Enthalpy of steam = 2675.53 and water = 418.991

From energy balance, we get 697.467 = 418.99*x + 2675.53*(1-x)

where (1-x) is the fraction of the water that converted to steam.

we get x = 0.8765

1-x = 0.1234

quality = 0.1234

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