DESIGN AND CONSTRUCTION OF CENTRIFUGAL PUMPS
Basic Pump Components
A typical Centrifugal Pump is shown in Figure 1. The main basic parts are described below.
The pump case or casing is the visible part of the pump. Most of the other parts are enclosed within it. It is usually made of cast iron, steel, plastic or other special materials. In the oilfield, casings on pumps operating at a pressure below 150 psig usually are made of cast iron. Pumps operating at higher pressure generally will have casings made of steel.
CENTRIFUGAL PUMP COMPONENTS
The impeller is the part of the pump that causes the liquid pressure to rise. It is firmly attached with a key and/or pressed onto the shaft. It rotates inside the case at the speed of the shaft.
The shaft rotates inside the case at the speed of the driver. It usually is made of steel. The section of shaft exposed to the seal or packing may have a sleeve made of a hard metal, such as tungsten carbide, to resist corrosion or wear at that point.
Bearings serve two functions on a pump:
- To support the shaft inside the pump casing.
- To prevent radial and axial movement of the shaft so that the rotating parts do not touch the pump casing. Thrust forces, developed as the impeller rotates, are the main cause of axial shaft movement. One or more of the bearings must be designed to withstand the thrust forces.
On small process pumps, the bearings may be contained in the pump casing. On larger pumps, the bearings are contained in housings located on one or both ends of the shaft. See Figure 2A
The bearings require lubrication. The bearing housing is partially filled with oil for lubrication and a drip-feed oil bottle supplies oil to the housing continuously. See Figure 2B
BEARINGS AND BEARING HOUSING
Mechanical Seals and Packed Shaft Seals
A mechanical seal or compressed packing is used to prevent liquid under pressure inside the pump from leaking out of the pump to atmosphere. Mechanical seals are used in most oilfield centrifugal pumps. It has two basic components:
- A stationery ring that is secured in the seal gland.
- A rotating ring that is part of the seal element attached to the shaft.
One of the seal rings is made of carbon; the other is made of hardened steel, ceramic or other special non-corrosive wear resistant material. Some seal manufacturers use a carbon stationary ring and others a carbon rotating ring. See Figure 3.
Packing often is used in lower pressure service, or in pumps handing abrasive liquids such as mud or slurry. Packing is composed of a series of flexible rings contained in a packing box. The rings are compressed by tightening the gland nuts. This squeezes the rings against the shaft and prevents liquid from leaking out. See Figure 4.
Mechanical seals generally require much less maintenance than packing, so they are used whenever possible. When they are used, liquid must be free of sand, dirt or other solid particles that can scratch the seal faces and cause leakage.
Alternative Seal Systems
A pump handling liquid hydrocarbon can cause a hazardous situation if hydrocarbon leaks out the pump seal to the surrounding atmosphere. Large pumps handling volatile or hazardous liquids are often equipped with a circulating seal oil system as shown in Figure 5. This system has two pump seals through which seal oil is continuously pumped to the seal chamber at a pressure higher than the pressure inside the pump. A pressure controller in the seal oil outlet line is set to hold this desired pressure.
CIRCULATING SEAL OIL SYSTEM
Figure 5 shows a multi-stage pump with seals at each end of the shaft. The balance line holds suction pressure on the pump side of both seals. Consequently, as long as the seal oil pressure is above pump suction pressure, leaking seals will result in seal oil leaking into the pump rather than pump liquid leaking to the seal oil system. Seal oil is a non-volatile liquid that does not contaminate the liquid inside the pump when it leaks into it. Some form of lubricating oil is often used for seal oil in hydrocarbon pumps.
The pump shaft connects to the driver with a coupling. Couplings transmit rotation from the driver shaft to the pump shaft. If a gearbox is between the driver and the pump, a coupling attaches the driver shaft to the inlet gearbox shaft, and another coupling attaches the outlet gearbox shaft to the pump shaft.
The couplings must be able to withstand the shock of a sudden change in pump load, or stoppage of the driver. They must be flexible enough to transmit power from the driver to the pump at high speed when the two shafts are not perfectly aligned.
It is almost impossible to perfectly align the two shafts, because the operating temperature difference between the driver and pump results in one shaft expanding slightly more than the other. The coupling must be able to withstand the resulting stresses enough to overcome the misalignment.
Wearing or Wear Rings
The liquid which has been discharged from the impeller is at a higher pressure than the liquid in the suction side and since clearance must be provided between the rotating impeller and the stationary casing, some of the liquid discharged leaks back to the suction.
To reduce the amount of leakage, various types of close clearance wear rings, one fitted to the impeller and the other to the casing are provided. Normally these companion rings are not in contact, but may come in contact because of bearing failure, temperature changes or excessive vibrations.