Module 5: Specialty Pumps and Pump Seals

Question 1

Axial Flow pump : summary

Answer 1

Axial impeller of 3-4 blades fitted in a cylindrical casing. Flow is axial (lengthwise) over the impeller.
Large amounts of slip in this pump.
Low suction head and low discharge head compared to other pumps

Question 2

Axial flow pump: construction

Answer 2

Impeller is connected to the prime mover by means of a stainless steel or carbon steel shaft. Due to its weight, the shaft must be properly supported by bearings
Prime mover may be located inside the pump casing (fitted with a sealed motor) or outside the pump casing (needs to be properly supported and sealed against leakage)

Question 3

Axial flow pump : operation

Answer 3

As impeller turns, fluid is drawn in and flows over the blades. Fluid is pushed in a direction parallel to the impeller shaft. Fitted downstream is a diffuser (stationary vanes) to redirect the fluid flow to the discharge line. The diffuser will contain a bearing to support the impeller and keep it centered in the housing

Question 4

Axial flow pump : flow rate

Answer 4

Flow rate depends on the pitch (angle) of the blades.
In some complex pumps, the pitch can be varied

Question 5

Axial flow pump : purpose

Answer 5

• Have the smallest dimensions among conventional pumps
• More suited for low suction head and high discharge
• Cannot draw from great depths or distances
• More rugged and can operate in a multitude of applications that require the movement of a high volume at a low discharge pressure
• Often used for raw water cooling, dewatering, and the rapid transfer of ballast water for ajustment of heel or trim

Question 6

Water ejectors : summary

Answer 6

Another method of removing fluid from a space
A vacuum pump that uses the kinetic energy of high pressure water or steam to suck in gasses or other liquids through a nozzle and discharge it overboard.
Essentially a Positive-displacement pump with no moving parts

Question 7

Water ejectors : materials

Answer 7

• The body is cast in brass or stainless steel and contains an inlet casing, mixing chamber, and diffuser
• A nozzle screws to the body at the inlet and is fabricated of high grade stainless steel

Question 8

Water ejectors : operation

Answer 8

• An ejector must be connected to 3 lines : a suction line, a discharge line, and a line for the high pressure motive (moving) fluid
• The fluid to be pumped enters the ejector at the suction connection.
• Motive fluid (generally high pressure water or steam) is injected through the inlet and as it passes through the nozzle, its pressure drops causing an increase in kinetic energy (high pressure, low velocity fluid is transformed into low pressure, high velocity). This causes a vacuum to appear at the suction connection, drawing in the fluid to be re moved
• The motive fluid and fluid to be moved mix, travel down the diffuser, are transformed into high pressure, low velocity and are ejected through at the discharge connection.

Question 9

Water ejectors : purpose

Answer 9

• No moving parts, require no maintenance, and are self-priming
• Commonly used in cargo applications to strip tanks, but also to remove water from bilges

Question 10

Pump shaft seals

Answer 10

Two types:
1. Packed gland
2. Mechanical seal

Question 11

1. Packed gland : construction

Answer 11

Consists of a series of packing “rings” placed around the shaft and compressed by a gland.
A lantern ring (a split ring/ H ring) is fitted between the rows of packing

Question 12

1. Packed gland : material

Answer 12

Packing is square in cross section and composed of hemp or a synthetic material
Packing material is specific to the application

Question 13

1. Packed gland : operation

Answer 13

• The packing must be lubricated due to friction between the packing and the pump shaft and keep the packing cool
• Lubricant (grease or the fluid being pumped) is injected into the space created by the lantern ring between the gland and the packing
• Grease is not ideal as may contaminate the fluid and needs to be constantly monitored
• Fluid being pumped could be pumped back from the discharge to the gland
• There will always be leakage with this type of seal : fluid travels in both directions - into the pump on one side and out of the pump (into the bilge) on the other

Question 14

2. Mechanical Seal : construction

Answer 14

Composed of 3 parts:
1. Seal faces : one rotating with the shaft and one stationary in the pump casing, These two faces are matched to each other and must be replaced as a set
2. Secondary seal : one to seal the rotating face to the shaft and one to seal the stationary face to the pump cover
3. Metallic cartridge containing a spring and the spring holder : used to provide axial force to maintain contact between the rotating and stationary seal faces and to keep the parts in alignment

Question 15

2. Mechanical Seal : stationary face

Answer 15

• Stationary face is fabricated of porcelain (generally if pumping water), stainless steel (generally if pumping fuel), or similar wear resistant material
• It is held in place against the gland follower (and leakage is prevented) by an o-ring that is fitted in a groove machined around the circumference of the ring.
• Leakage is stopped between the gland and the seal chamber by a gasket

Question 16

2. Mechanical seal : rotating face

Answer 16

• Rotating face is normally fabricated of carbon graphite because it is hard, resistant to wear, and self-sealing.
• An o-ring is fitted between the rotating face and the shaft to hold it in place and to prevent leakage down the shaft

Question 17

2. Mechanical seal : comments

Answer 17

• The metallic cartridge is fitted on the shaft and the spring forces the fixed and rotating faced onto each other
• It is essential that the seal faces be flat and polished, be installed perpendicularly to the shaft, and the spring force must be sufficient to maintain positive contact of the faces
• Disadvantages : must be installed properly, expensive, must be taken out of service to repair/replace the seal