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Pressure Driven Pumps & Open System Return
A new choice to ensure proper condensate removal is the Pressure Operated Pumpless Condensate Unit. This type of pump uses steam or compressed gas (air) as a "motive force" to produce the differential pressure needed to transfer the condensate. There are no impellers, seals, or electric motors used with the Pumpless Condensate Unit. ITT Domestic offers this type of unit under the model name PCC.
The Pumpless Condensate Unit uses a float mechanism to operate the pump cycle. This float automatically connects to an exhaust valve (normally open) and a steam/air valve (normally closed). Condensate flows by gravity through the inlet check valve into the pump body. Air vents through the exhaust valve and allow the pump to fill. As the pump fills, the float rises. At a predetermined level, the float mechanism opens the steam valve and closes the exhaust valve, introducing steam or air into the pump. As the pump pressurizes, the inlet check valve is forced closed, and the condensate is pushed out of the outlet check valve and into the condensate return piping system. When the pump empties, the float switches the position of the steam and exhaust valves, and the cycle begins once more.
The Pumpless Condensate Unit does not function as a steam trap; the pump cannot discharge condensate while holding back steam. The ability of the pump to operate and the capacity it can handle are functions of:
The differential pressure between the motive force and the pressure in the condensate return piping
The check valve sizes
The height difference between pump and receiver
The operation allows the pump to accept
condensate during the filling cycle only. The pump discharges and
condensate stores elsewhere until the discharge stroke is complete. For
this reason, a receiver must be installed before the pump. The receiver
should be sized for the amount of condensate that will form during the
pump's discharge cycle. The receiver stores the condensate, reducing the
possibility of backing condensate into the steam space of a heat
exchanger. The vent line on the receiver must be sized to handle the
flash steam created as hot condensate discharges from the steam trap(s)
into the receiver. A small vent line creates pressure inside the
receiver, raising back pressure against the trap. The height of the
receiver relative to the pump is important. The pump fills up with
condensate fed from the receiver. A minimum distance of 12" from the
bottom of the receiver to the top of the pump is required to provide the
pump's rated capacity.
At distances greater than 12", the capacity of the pump increases since
the rate at which condensate flows from the receiver to the pump is a
function of static head.
The size of the inlet and outlet check valves also influences the pump's capacity. Standard options are l"x1", 2"x2", and 3"x2" (inlet size by outlet size). The larger the valves are the higher the capacity.
Differential pressure between the motive force and the return line has the largest effect on pump capacity. The larger the differential pressure is the higher the capacity. The Pumpless Condensate Unit operates similar to a steam trap concerning differential pressure and capacity. The larger the difference between motive pressure and system back pressure, the higher the capacity of the pump.
The Pumpless Condensate Unit is sized to handle the condensing rate of the heat exchanger. The maximum motive pressure recommended is 125 psi (100 psi is usually more than enough; it also eliminates the possibility of water hammer inside the pump that is sometimes present with a 125 psi motive force). A steam trap is installed at the inlet of the motive force to remove condensate that forms while idle steam condenses in the line during the filling cycle of the pump. The vent line from the Pumpless Condensate Unit is tied into the vent line of the receiver. An overflow pipe, installed below this tie-in point, allows condensate to drain should the receiver be undersized or the pump fail to cycle.
This overflow connection is important in preventing condensate from backing up into the steam space of the exchanger and also warning the operator that possible problems exist in the system.
A pressure gauge should be installed on top of the Pumpless Condensate Unit to help detect any problems, and to help estimate condensate flow and back pressure. This gauge assembly should include these components:
- Gauge
- Pigtail siphon, used to prevent live steam from entering the bourdon tube which could cause damage to the gauge
- Isolating valve (normally closed), limiting wear and tear on the gauge when the operator is not using it
- Pressure snubber, used to dampen pressure shocks experienced with the sudden introduction of motive force. During the fill cycle, the pump vents to atmosphere and the gauge should read "O." As the pump cycles, the gauge will read a pressure reflecting the return pipe system pressure. The gauge is also useful to display the cycle's length and frequency, in addition to the back pressure on the system.