A maintenance drain line is typically consists of a block valve (ball or gate subject to service) and a globe valve. Block valve is provided for isolation purposes while globe valve for throttling to avoid excessive flow to closed drain system. Drain minimum line size is typically 2" (typical but may be different elsewhere). Sudden opening of 2" block valve would results excessive flow to close drain system and potentially leads to high noise level, acoustically induced vibration, erosion-corrosion, etc. A globe valve is provided on the drain line to limit sudden large flow into closed drain line. Other than above mentioned consequences, pressure drop across globe valve will results low temperature (lower than zero degC) due to JT effect. Manual block valve located downstream of globe valve would experience similar temperature and leads to ice form at valve body, stem, shaft, etc. This would disable re-closure of the manual block valve. Thus, it is always advisable to locate manual block valve upstream of globe valve.

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