Page 223 - Valve Selection Handbook

210 Valve Selection Handbook

flow, the valve should be mounted as close as possible to the pressure
source.
If the piston of the main valve is undamped, flow disturbances in the
inlet pipeline such as caused by a single elbow or an equivalent restric-
49
tion can already lead to valve chatter. In the case of full-bore main
valves, stability may be achieved by limiting the length of the inlet
pipeline to an L/D of 5. This requires an excellent rounded entry to the
pipeline. Where longer pipe runs are required, the inlet pipe size should
be one size larger than the valve inlet and be used with a concentric
reducer below the valve. In less than ideal installations, frictional resis-
tance to the piston movement can completely suppress valve resonant
chatter. This has been achieved in the valve shown in Figure 5-36 by a
pressure-activated drag device between piston and cylinder.

Stable operation of valves with modulating pilots. Pilot-operated
pressure relief valves with modulating pilot are more forgiving to unfa-
vorable installation conditions than those with on/off pilot, primarily
because of the valve opening in proportion to demand.

Several types of pilots are available:

Flowing type: (a) Not preferred unless the process medium is relatively
clean, (b) Preferred for steam service. Flowing type allows the pilot to
warm up internally to prevent sudden steam condensation inside the
pilot that would lead to pilot instability and erratic action.
Non-flowing pilot is generally preferred.
Pop action: (a) Preferred if rate of pressure rise is high, or (b) if process
medium can form solids after expanding through the pressure relief
valve that could plug or partially plug the discharge header. Example:
CO 2 forms dry ice particulate upon expansion.
Modulating action: (a) Preferred when required discharge capacity can
vary greatly, (b) also preferred for operation in liquid service, (c) to
ensure valve stability if the valve is oversized or, within limits, under-
sized due to underestimating the inlet pressure loss when sizing the
valve, or (d) when the state of process phase can vary. Example:
Oil/gas separator that is liquid-flooded.

Consult manufacturer if fluid carries solids in suspension or when
freezing or condensation of the lading fluid at ambient temperature is pos-
sible. Do not use pilot-operated valves in abrasive or viscous services