The liquid is clean, free from sediment and chemical precipitation, and compatible with seal materials.
The temperature is above 32F and below 160F.
The suction pressure is below 75 psig.
The liquid has adequate lubricating qualities.
The liquid is nontoxic and nonvolatile.
EXTERNAL LIQUID LUBRICATION. - When the liquid being pumped contains solids or is otherwise not compatible with packing materials, an outside supply of seal liquid should be furnished. In general, external-injection liquid (from an outside source) is required when the following conditions exist.
Liquid being pumped contains dirt, grit, or other impurities.
The temperature of the pumped liquid is below 32F or above 160F.
The liquid being pumped has nonlubricating properties.
The liquid is toxic or volatile.
The suction pressure is above 75 psig, vacuum, or high lift.
Mechanical seals are preferred over packing on some applications because of better sealing qualities and longer serviceability. Leakage is eliminated when a seal is properly installed, and normal life is much greater than that of packing on similar applications. A mechanical shaft seal is supplied in place of a packed stuffing box when specifically requested. The change from packing to an alternate arrangement may be made in the field. Conversion kits may be ordered from the manufacturer.
SINGLE SEAL. - Pumps containing single mechanical seals normally use pumped liquid to lubricate the seal faces. This method is preferred when the pumped liquid is neither abrasive nor corrosive. If the liquid being pumped is not suitable, an external flush should be provided. (See "External Liquid Lubrication" above.)
DOUBLE SEAL. - A double mechanical seal consists of two single seals mounted back to back and a suitable sealing liquid that is introduced into the seal chamber. The sealing liquid (preferably clear water) is injected into the box at a higher pressure than exists at the entrance to the seal cavity on the pump side. The pressure differential isolates the sealing faces from the pumped liquid. Double mechanical seals are normally preferred in pumps handling sewage, slurries, or any other solids suspended in the pumped liquid.
Sealing liquid that is introduced through the tap in the seal cavity provides lubrication for the double seal. The sealing liquid pressure must always be higher than the pressure on the seal closest to the suction side. If sufficient sealing pressure is not maintained, the pressure within the pump can force open the lower seal and allow the pumped liquid to enter the box. This can damage the seals.
Two methods are used to provide sealing liquid to the stuffing box. The first method uses a pressure line installed from a tap on the discharge nozzle to the tap in the stuffing box cartridge. A filter is installed in the line to trap the solid particles. The filter must be capable of screening out all particles above 25 microns in size. Since the liquid is bypassed from the high-pressure (discharge) side of the pump and dead-ended in the stuffing box cartridge, there are no problems in maintaining a sufficient pressure differential, provided the filter is not clogged. The second method uses clear, clean water supplied from an external source. City water can be used if there is an air break between the water supply and the water being provided to the pump. Various municipal ordinances require this break to prevent contamination of the city water supply.
Operating conditions vary so widely that to recommend one schedule of preventive maintenance for all centrifugal pumps is not possible. Yet some sort of regular inspection must be planned and followed. You should maintain a permanent record of the periodic inspections and maintenance performed on a pump. This procedure will assist you in keeping the pump in good working condition and prevent costly breakdowns.
One of the best rules to follow in the proper maintenance of a centrifugal pump is to keep a record of actual operating hours. Then, after a predetermined period of operation has elapsed, the pump should be given a thorough inspection. The length of this operating period varies with different applications and can only be determined from experience. New equipment, however, should be examined after a relatively short period of operation. The nextContinue Reading