Tuesday, December 21, 2010

PUMPING SYSTEMS ON CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL



A pump is used to move liquids from lower pressure to higher pressure.


CENTRIFUGAL  pump is basically a kinetic pump which increases the flow when through the pump—it centrifuges the liquid into the discharge line.

If the flow into the pump suction is less than the delivery the pump will cavitate causing gassing and loss of suction. They are resistable to solids in cargo. The big disadvantage is its inability to evacuate air/ gas from its casing. The pump casing must be filled with liquid before starting—the pump must be stopped to do this. When the impeller starts to turn the liquid is driven to the periphery of the housing by centrifugal forces. This results in a positive pressure on the outside of the impeller and a negative pressure in the centre.


Centrifugal pumps can be operated with dischg valve closed as long as the pump does not grow too hot. The last part of dischg is more effective with pump running at low RPM and against a throttled delivery valve. For hot liquids and those which flash into vapour at pressures below atmospheric pressure, the liquid must flow into the pump suction under positive pressure.


Never run a centrifugal pump dry. The pump bearing temp must not exceed 90C. No liquid will be delivered if the pump rpm is too less when the dischg head is too high or suction lift is too high . The viscosity of liquid cannot be too high. There cannot be any leaks on the suction side pipelines or air leak in the shaft seal or vaporisation in the suction line. Never try to reduce pump capacity by throttling the valve on the suction side.


The casings of centrifugal pumps without a self priming system should be vented of air or gas and primed full of liquid before starting the pump.

The pump suction valve should be full OPEN and a flow of liquid should be available to the pump suction when the pump is started.

Variable speed pumps should be started with the pump discharge valve closed and as the pump speed is increased the discharge valve should be progressively opened to maintain an appropriate discharge head.

Constant speed pumps should be started with the discharge valve only slightly opened and the discharge valve further adjusted to maintain an appropriate discharge head once the pump has been started.

Control of the capacity of a centrifugal pump can be obtained either by throttling the pump discharge valve (and artificially increasing the total head) or, if possible, by restricting the pump's speed or by a combination of both of these methods.  In general for bulk cargo operations it is preferable to throttle the discharge valve only and so maintain the full power of the pump to overcome the head resistance of the terminal piping system.

This is most important where the pumps are being run at different speeds as the pumps running at the low speeds may have their pumping rates severely restricted by the system discharge head. Certain times it may not discharge at all and are called sleeping pumps.

The pump suction valve must always be full OPEN when the pump is running and this valve must not be throttled to control capacity during operation.  

In general, the suctions of several centrifugal pumps should not be cross‑connected and each pump should take suction individually from its own main suction line.

When the liquid level in a tank being pumped out reaches a low level the pump suction condition should be maintained at an adequate value by either cracking open a tank with a higher liquid level or reducing the pump speed.  Inadequate suction conditions (high vacuum) will cause cavitation of the pump and possible gassing up and loss of pump suction.  


DEEPWELL pumps—are centrifugal pumps. They are self priming as long as the suction well is full. 85% if centrifugal pump pumping problems occur on the suction side. This has been eliminated on deep well pumps which do NOT have a suction pipe.


They are variable speed pumps as their rpm can be manually adjusted from zero to maximum designed speed using the pump’s control lever. The pumps’ hydraulic system (which incorporates a STC ) is so designed that the maximum design speed of the pump cannot be exceeded regardless of the pump suction and discharge conditions. They are thus different from the conventional variable speed centrifugal pumps and over speed trips are neither required nor fitted .


When your run a Framo pump at full speed in a tank of low SG cargo the hyd pressure will be less than a high SG cargo. For the same speed any variation of the pump’s suction and discharge heads will also alter the hydraulic driving pressure.

Increasing the speed control lever setting will increase the driving pressure and reduction will reduce the driving pressure.

When operating several pumps in parallel , the driving pressure of each pump should be about the same value, lest there is an overflow.

Framo pumps are self priming pumps , hence it is not necessary to vent the pump before starting.

Since there are no suction lines for deep well pumps, there is no need to consider flow velocities in suction piping .Only the velocities in cargo discharge piping system need be considered when operating at high throughput.

The Framo deep well pump may be started with the discharge valve open. However, if other pumps are discharging to the same line, or if there is a standing head of cargo, assuming there are no non-return valves like on Japanese deep well pumps , the opening of this pump’s discharge valve will permit liquid in the piping system to pass into this tank unless the pump is running and discharging. The standard procedure shall therefore be to start the pump with the discharge valve CLOSED.


RECIPROCATING pumps can apply unlimited delivery pressure –the limit being set by the size and solidness of the pump. It does not have to be filled with liquid prior pumping starts. Air/ gas is evacuated from the suction side of the pump.

The pump delivers pulsating flow –double acting pumps are less pulsating . In order to smooth en the flow and to make the flow constant , air vessels are uses on the suction/ delivery side –they act as accumulator dampers as long as they have air inside them for the air cushion. The acceleration is now restricted to the space between the air vessels. If the acceleration and deceleration inside the suction pipes are too much, the result is cavitation and vibrations.

They are sensitive to solids in cargo. Reciprocating pumps are too slow. The pump is primed when in motion. Before starting a stripper pump make sure all the suction side and delivery side valves are open.

The suction and discharge valves of a positive displacement pump must always be open before starting the pump and must remain open until the pump is stopped.

These pumps must not be operated in excess of their designed speed and particular care must be taken to avoid these pumps over speeding when they lose suction.


SCREW PUMPS —they are self priming. The rotating elements are axially balanced and there is no metal to metal contact within the pump between the screws or the body .

They can handle virtually any non homogeneous liquid regardless of viscosity and abrasiveness—molasses, caustic soda, sea water , bitumen, clay slurry etc. generally used on a chemical tanker as a booster pump for molasses.

Screw pumps are rotary, positive displacement pumps that can have one or more screws to transfer high or low viscosity fluids along an axis typically screw pumps have two or more inter meshing screws rotating axially clockwise or counterclockwise. Each screw thread is matched to carry a specific volume of fluid The capacity of screw pumps can be calculated based on the dimensions of the pump, the dimensions of the surface of the screws, and the rotational speed of the rotor since a specific volume is transferred with each revolution.

Screw pumps and piston pumps must have a bypass line and safety valve to protect the delivery line which may burst if the shore tank valve is not open. Positive displacement pumps like piston/ screw/ gear pumps are never started with delivery valve shut. The viscosity of the fluid transferred and the lift required may affect the speed and power required. Indicators of pump malfunction include decrease in flow rate or increased noise.

The efficiency of screw pumps requires that each rotor turns at a rate that allows each cavity to fill completely in order to work at full capacity--.
Low pulsation for smooth, quiet operation.
Direct-drive for design and operational simplicity. 
Low shear for protection of the pumped fluid.
Hydraulic balance for minimal deflection and wear.
Cartridge-type rotating assembly for easy servicing.  
Integral relief valve to eliminate external valves and piping
Built-in heat jacket to facilitate cold start.
Customized ports to adapt to the best piping arrangements for the system.
Selection of materials for the longest service life.


It can handle a highly viscous liquid of up to 1,000,000 mPaS like treacle molasses of high Brix value , as well as a material that is almost non-fluidal. The liquid can be transferred smoothly without pulsation, stirring, or foaming.



Furthermore, the flow rate can be controlled easily by adjusting the pump speed. The self-priming capability is maximum 9 m, making the pump ideal for assisting the deep well pump by sucking from underneath –from the cargo tanks Drive is usually Framo powered –they are able to withstand high temp and back pressures


EDUCTORS cannot strip cargo to shore due to its limited capacity to work against back pressure—the drive flow has to be maintained. So you have to do internal stripping.

Eductors must be operated at the full designed pressure—or its efficiency becomes less. Open the eductor suction valve last and shut it first —to prevent back flow, and also suction lines may not be strong enough to hold the pressure. 


When stopping an eductor the eductor suction valve should be closed before reducing the driving pressure and the tank suction valve should remain open until the eductor is stopped in order to avoid shutting down the piping system with a 'locked in' vacuum. 

If, during use, the eductor driving pressure falls below the operating pressure range, the eductor suction valve should be closed to prevent backflow of the driving liquid. The tank suction valve must not be used to prevent back flow as these valves and the suction piping may not be designed to withstand such high pressure.


Also portable eductors must be installed as low as possible.

Operation of the eductor against a higher discharge head than the design discharge head  has the effect of reducing the eductor's suction capacity.

The eductor uses an internal nozzle to produce a vacuum within it. A driving liquid leaves the nozzle at high velocity producing a vacuum which draws in the liquid to be pumped, and expels it by transferring its kinetic energy. It can pump liquids vapour and impurities , as long as the nozzle does not get restricted. However a disadvantage is that the driving liquid must be compatible with the liquid to be pumped.


VARIABLE DISPLACEMENT PUMPS The displacement can be adjusted to increase or decrease the amount of fluid pumped.



A common variable displacement pump is the axial piston pump. This pump has several pistons ( usually odd number ) in cylinders arranged parallel to each other and rotating around a central shaft. A swash plate at one end is connected to the pistons. As the pistons rotate, the angle of the plate causes them to move in and out of their cylinders. A rotary valve at the opposite end from the swash plate alternately connects each cylinder to the fluid supply and delivery lines.


By changing the angle of the swash plate, the stroke of the pistons can be varied continuously. If the swash plate is perpendicular to the axis of rotation, no fluid will flow. If it is at a sharp angle, a large volume of fluid will be pumped. Some pumps allow the swash plate to be moved in both directions from the zero position, pumping fluid in either direction without reversing the rotation of the pump. By adjusting the position of the swash plate, variable displacement axial piston pump can vary flow smoothly, as flow is proportional to displacement.


At maximum angle, displacement is at its highest because the pistons travel at maximum length. When the angle is reduced, piston travel shortens, reducing displacement. If flow remains constant, a motor runs faster, but torque is decreased. Torque is greatest at maximum displacement because the component of piston force parallel to a swash plate is greatest .

The FRAMO deepwell pump power pack pumps are of axial piston type , swash plate design with variable displacement. The pump displacement ( swivel angle ) is hydraulically controlled via the pressure regulator of each main pump.  If the oil consumption is increased the swash plate is pushed to increased swivel angle until a new balance is reached between oil delivery and consumption. By this method oil delivery from the hydraulic pump will always be the same as oil consumption from hyd motors. In variable displacement pumps , by changing the angle of pump axis ( swivel angle ) delivery change can be effected. (angle between cyl block and shaft is varied ). Swivel angle of the pump is hydraulically controlled with spring centring via the control cylinder. A bigger angle will give more oil flow, but it will also require a bigger torque on the drive shaft to maintain the same pressure.

SLIDING VANE PUMP

ROTARY LOBE PUMP



CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )
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