Liquid Oxygen Pumps
The Liquid Oxygen Pump is a single stage, single acting piston pump. It is used for filling oxygen into cylinders up to a pressure of 150 Kgs./cm2.
The pump is designed for assembly in air separation unit that works by pumping liquid oxygen and gasifying the same in Heat Exchangers for final filling as gas in cylinders. Control of liquid feed is not necessary, because the pump is designed in accordance with the plant size and the liquid produced is constantly pumped off.
The drive unit is similar to any reciprocating machine with the crank shaft, flywheel, connecting rod, cross head etc. as can be seen from the drawing supplied with the manual.
The Liquid Oxygen Pump consists of a stainless steel inside liner with liquid in-let and evaporated gas outlet port. There are no valves on theses ports, which are closed by the piston itself on the pressure stroke. The third outlet is the main discharge outlet with the two non-return ball valves. The two valves remain firmly closed during suction stroke due to high pressure in partly filled cylinders.
To ensure that these valves are fully closed, a positive pressure of about 40-60 kgs/cm2 must be maintained on it. When a fresh batch of cylinders is taken for filling open the manifold valve slowly or uses a spare batch of cylinders to ensure a positive pressure on these valves. Most mul functions of the pump are due to these valves not closing properly.
When the pump is operated liquid oxygen from the main condenser enters the outer jacket of the pump. Some of this liquid evaporates due to heat produced in pumping and the vapor is passed out through the upper port back into the upper column as gas. The main stream of liquid oxygen is taken into the pump cylinder and compressed out by piston in the pressure stroke. This high pressure liquid oxygen passes through two non-return valves into the heat exchangers and then to the cylinder filling rack.
If the molecular sieve is not working properly, some carbon dioxide and moisture will condense into the condenser and will travel to the L.O Pump inlet where a filter is provided. The solid CO2 may block the filter and the pump will not operate efficiently.
In such an event the L.O. Pump should be defrosted. This way, the solid carbon dioxide will be removed and the pump will now work satisfactorily unless there is a continuous carry over of CO2. Drain a little L.O in a double walled Dewar's flask and check against light. If the liquid is turbid, there are CO2 crystals in it. If it is clear the fault is elsewhere.
During dispatch of the plant, the drive unit along with the inside pump is detached from cold box. The drive unit should be placed on its foundation. The bolts connecting the drive unit mounting flange to the cold box should be loosely tightened. This piston rod is to be rightly connected to the cross head. Now, the alignment should be such as the piston rod in the dead center of stuffing box. This can be checked by means of a feeler gauge, around the piston rod in stuffing box casing. This clearance should be same for any forward and backward position of piston. When this is centralized, the piston moves smoothly. The connecting bolts can be tightened without upsetting alignment and the drive unit is to be grouted.