STEAM TRAP CAPACITY
A thermostatic steam trap can be likened to a control valve with a condensate capacity (Cv) that varies from full-open to
full-closed as the stem travels up and down. Manufacturers publish capacity data based on a 10 °F sub-cool, which is a generally
accepted benchmark for comparing traps' maximum capacity. Liquid condensate capacity is defined as in equation :
where q is the flow in gal/min, P1 is the upstream condensate pressure in psia, P2 is the downstream pressure in psia, and
G is the specific gravity. Therefore we solve for equation .
where W is the flow rate measured in lb/h, 60 is min/h and 8.337 is cold-water (25 °C) density in lb/gal.
There is a limit to P2. As the condensate moves through the trap, its pressure is reduced and the hot condensate flashes.
When the downstream pressure is approximately 50% of the upstream pressure, the pressure drop becomes critical. At this point
the flashing steam reaches sonic velocity, which chokes off any additional flow through the steam trap. In this case the downstream
pressure P2 is the vapor pressure of the flashed condensate.
The test trap discussed in this article is the Nicholson CDS 204 with –B bellows. It has a 5/16 in. orifice (3/4-in. inlet)
with a cold water Cv of 1.59. Table 1 indicates the theoretical capacity when passing 80 °C water and the published 10 °F
sub-cooled condensate for 20-psig steam. A theoretical capacity at a 2 °F sub-cool is also shown. This calculated capacity
is actually larger than the actual capacity because it assumes that the trap is fully open. At a 2 °F sub-cool there is not
enough temperature difference to open the trap fully, leaving the trap partially open.
Table 1. Theoretical CDS steam trap capacity (wide open Cv = 1.59; steam temperature = 20 psig).
SIP systems for vessels and associated piping are highly engineered systems. There are several different arrangements used
by the biotech industry to purge air, heat up the vessel, and maintain a steam temperature of 121 °C for a minimum of 15 min.
Figure 2. Steam trap piping. Arrangement #1 has a three-way valve. Arrangement #2 for smaller systems has no valve.