Positive Displacement Reciprocating Pumps Engineering Essay
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Published: Mon, 5 Dec 2016
Reciprocating pumps are positive-displacement pumps in which water, fluids, slurries (depending on their abrasiveness) and even solid materials is moved by means of a plunger or a piston that reciprocates back and forth in a horizontal or vertical direction inside a cylinder; each stroke displaced a quantity of liquid regardless of the resistance against which pump is operating. Reciprocating pumps have a fluid-handling portion, commonly called liquid end cylinder, which has:
- A displacing solid called a plunger or piston.
- A container to hold the liquid called the liquid cylinder.
- A suction check valve to admit fluid from the suction pipe into the liquid cylinder.
- A discharge check valve to admit fluid from the liquid cylinder into the discharge pipe.
- Packing to seal tightly the joint between the plunger and the liquid cylinder to prevent the liquid from leaking out the cylinder and air from leaking into the cylinder.
It must also have a driving mechanism to provide motion and force to the plunger or piston. The most common driving mechanism is reciprocating steam engine and a crank and throw device. Pumps using a steam engine are called direct-acting steam pumps while those pumps using crank and throw device are called power pumps. Power pumps must be connected to an external rotating driving force such as an electric motor, steam turbine, or internal combustion engine.
Reciprocating pumps are classified by their field of application whether direct or indirect acting; a simplex, duplex, triplex, quintuplex, septuplex and nonuplex; a single or double acting; a High pressure or Low pressure; and a Vertical or Horizontal Pumps. The term simplex or simple pump means it has only one liquid cylinder, on the other hand duplex or double pump is an assembly of two simplex pumps placed side by side on the same foundation. In a single-acting pump the liquid is drawn into the cylinder on the first or suction stroke and forced out on the return or discharge stroke while the double-acting pumps, each stroke serves both to draw in the liquid and discharge the liquid; as the other end of the cylinder is emptied the other end is filled and on the second stroke emptied end is filled while the other end is emptied vice versa every stroke. When the discharge pressure is high the discharge volume of the liquid is relatively small; it is the main characteristic of a steam and power driven reciprocating pumps. Therefore the discharge pressure of this kind of pump is inversely proportional to discharge volume of the liquid. Since the plunger and piston type pumps are the basic components of reciprocating pumps both which have the desirable characteristics of maintaining high volumetric efficiency at any desired flow rate, which gives greater flexibility in system design. The degree of abrasiveness of the slurry to be pumped will decide the type of pump to be selected.
Piston type pumps are considered for low-abrasion mixtures such as coal slurries and mud; they are more suitable for high-volume flows and low and moderate pressures up to 2,000psi gage solids sizes limited to 8 meshes or lower. While plunger type pumps are considered for high-abrasion fine mixtures such as magnetite, silica, and sand slurries. This type is considered for heavy duty and high pressures above 2000psi gage. The pressure developed by the pump is proportional to the power available at the crankshaft. This pressure can be greater than the rating of the discharge system or pump. When the pressure developed is greater than those ratings, a mechanical failure can result. To prevent this, a pressure relief device should be installed between pump discharge flange and the fist valve in the discharge system.
TYPES OF RECIPROCATING PUMPS
As stated above reciprocating pumps are classified by their field of application whether direct or indirect-acting. Indirect-acting Power Pump is a constant-speed, constant torque, and nearly constant capacity reciprocating machine whose plungers or piston is driven by a crankshaft from an external source. These positive displacement machines which at constant speed, deliver essentially the same capacity at any pressure within the capability of the driver and the strength of the pump. The inherently high efficiency of a power pump is almost independent of pressure and capacity and is only slightly lower for a small pump than a larger pump.
Thus power pump is most useful in the field of high pressure and low capacity, where its efficiency more than offsets the high initial cost. These pumps in other applications has an advantage and disadvantage, at varying pressure the delivery is constant and this kind of pump also serves as a metering device. While in other applications this creates a control problem to be met by varying the speed, by-passing at constant speed, or intermittently loading and unloading the pump. Power pumps are supplied with two, three, five, seven and nine plungers. They are offered with different diameter and number of plungers to cover a range of displacement with strokes from 2 ½ to 9in.Power pumps can handle slurries consisting of 65%weight of solids. Horizontal power pump is usually offered as triplex and quintiplex pumps with single acting plungers and is widely used pump for water flooding and saltwater disposal in the oil fields, for gathering and product pipelines, and for small hydraulic testing. In oil field use the pressure range is extended up to 1500psi. Pressure characteristics must be at least equal to the sum of the resistance to: vapor pressure of the liquid in the pump chamber; the suction lift when the liquid is below the pump level; the pressure required to lift the suction valve and overcome the resistance of its spring; liquid friction in the suction pipeline; the forces required to accelerate the liquid in suction pipeline; hydraulic losses in the pump.
Power pumps of slower speed are used for viscous hot liquids, oil refinery service, slurries, and urea service. Higher speeds are used where weight is a consideration, as in marine service and hydraulic press service of high pressures.
The direct-acting steam pump is a very flexible machine. It operates at any point of pressure and flow within the limitations of the particular design. The speed of, and therefore the flow from, the pump is controlled from stop to maximum by means of throttling the steam valves supply and thus can be done automatically or manually. Maximum speed is primarily dictated by the frequency in with which the valves will operate smoothly close or open. The pump in this system will operate against any pressure that is imposed by the steam from zero to maximum and is determined by the strength of the liquid end. In some particular application developed maximum pressure is determined by the steam pressure exerted on the system and the ratio of the cross-sectional area of the steam piston or plunger against liquid piston or plunger areas. The steam consumption of direct-acting pumps will vary from 200lb of steam per water (hp.h) for small pumps at light loads to as a little of 50 lb of steam per water (hp.h) for large pumps operating at 350 to 450 psi steam pressure. For pumps operating with an exhaust pressure greater than atmospheric, the steam consumption is increased proportion to:
where p=initial pressure at steam cylinder inlet (psig); b=exhaust pressure (psig).
When using a superheated steam deduct 1 percent for each 10oF of superheat. The performance of the steam pump is expressed as duty in ft.lb (Joule) of work done per 1,000 lb of dry steam.
Brake Horse Power of reciprocating pump can be found using this formula:
where Qd=pump delivered capacity (gpm); Plp=is net liquid differential
pressure (psi); ME=mechanical efficiency of the system.
Comparing to the total discharge pump capacity the displaced capacity inside the liquid cylinder is not equal to actual discharge of the pump, there difference is called slip (S). Therefore pump delivered capacity can be written as:
Where D=is the calculated capacity with no loss due to slip. The fluid includes liquid, entrained gases, and solids at specified condition.
S= (slip) the capacity loss as a percentage of suction capacity.
for single-acting piston or plunger,
for double-acting piston or plunger,
Where A = area of piston or plunger, in2; a=area of piston rod (negligible), in2; m=number of plunger or pistons; n=rpm; s=stroke,in.
(To take note that n for power pumps ranges 300 to 500 rpm, depending on capacity, size and horsepower and plunger speed of 140 to 150 fpm.)
On the other hand at that 1 revolution of a steam pump is defined as one complete forward and reverse stroke of the piston.
Ve1=volumetric eff. loss; B1=stuffing-box loss (negligible); V1=valve loss (2 to 10% depending on valve design and conditions).
Where Ve= the ratio of discharge volume to suction volume of fluid as a percentage proportional to ratio r and differential pressure.
is the ratio of internal volume of fluid between valves when the plunger or piston at the top of its back stroke (C+D) to plunger displacement D.
Ve is higher due to fluid compressibility because at discharge pressure volume is not readily been measured and it is taken on suction pressure. For this instance compressibility loss becomes important for water over 6,000 psi. The approximate percentage loss for 1,000psi at 68oF is water,0.29;lubricating oil,0.44; kerosene,0.51; gasoline,0.70; ammonia, 0.98; n-butane, 2.4; propane, 3.0.
In the actual practice the force on the steam piston should be greater than the force opposing to it on the liquid piston cylinder by a considerable amount. These include stuffing box friction, friction between rings and cylinder of both liquid and steam ends, and the operation of the valve. In quantitative form it is written as Psds2>Pldl2, where dl=is the diameter of liquid piston cylinder and ds=is the diameter of steam piston cylinder. These losses are determined by test and are accounted for size calculations by the introduction of mechanical efficiency figures.
Mechanical Efficiences are expressed as a percentage, with 100 percent being a perfect balance of forces acting on the steam and liquid pistons. Since efficiencies of two identical pumps may vary, with stuffing box and piston-ring packing tightness, applying the mechanical efficiency factor, the formula is written as: mechanical efficiency inside cylinder, expressed as a decimal where
Ps=net steam pressure; ds=steam-piston diameter;Pl=net liquid pressure;
dl= liquid-piston diameter.
The above formula is used to determine the minimum size of the steam piston when the liquid piston size has already been selected and net steam and net liquid
The efficiency of the long stroke pump is greater than the short stroke pump. Although mechanical eff. varies with stroke length, any two pumps of the same size are capable of the same efficiency. Table below shows a typical mechanical efficiency in percentage which can be used to determine the required steam-end size.
Stroke,in 3 4 5 6 8 10 12 18 24
Piston pump 50 55 60 61 65 70 70 73 75
Plunger pump 47 52 57 61 61 66 66 69 71
Since the most operating problems with pumps of all types are caused by failure to supply adequate suction pressure to fill the pump properly. Today the steam pump industry uses the term NPSHR net positive suction head required to define the head or pressure required by the pump over its datum, usually the discharge-valve level. This pressure is needed to overcome friction losses in the pump, overcome the weight and spring loadings of the suction valves, and create the desired velocity in the suction opening and through the suction valves. The NPSHR of a steam pump will increase as the piston speed and capacity are increased. The average steam pump will have valves designed to limit the NPSHR to 5psi or less at maximum piston speed. If NPSHR is not meet by an absolute pressure in the suction system minus the vapor pressure, the pump will create cavitations. Cavitation is the change of a portion of the liquid to vapor, and its causes a reduction in delivered capacity, erratic discharge pressure, and noisy operation. Likewise the NPSHA is also the very important criteria in designing such a pump it is the summation of static head, atmospheric head, minus lift loss, vapor pressure, velocity head and acceleration head in feet available at the suction connection centerline. Other important factor considered in reciprocating pumps is the Acceleration head. Acceleration head it is the energy required to keep the fluid in the suction pipe from falling below vapor pressure. To overcome this phenomenon we use the following empirical formula:
where Ha=liquid pump to produce required acceleration;L=Actual suction pipe length;V=mean flow velocity in suction line;C=Pump constant factor: 0.20=single duplex,0.20=duplex single,0.40=simple single acting,0.115=duplex double acting,0.066=triplex single or double acting,0.04=quintuplex single or double acting,0.028=septuplex single or double acting; k= liquid factor:2.5=high compressible carbons,2.0=most hydrocarbons,1.5=water,amine,glycol,1.4=deaerated water;g=gravity.
Advantage and Disadvantage of Reciprocating Pump
Below summarizes the advantage and disadvantage of reciprocating pump:
- Lower capacities up to 600gpm and high pressure up to 2000psi
- The lower the positive displacement speed, the lower the NPSHR
- Alternative to a centrifugal operating away for best eff. point
- Constant flow regardless of pressure, pressure and flow are independent to each other
- Variable capacity is achieved by varying speed
- Also used for metering application and if application has a variable pressure
- Self-priming with the right suction conditions
- High mechanical eff.
- Can be used on certain types of fluids containing solids that would quickly erode other pumps
- Discontinuity in the discharge flow
- Prone to flow separation at low pressure in the system
- Steam consumption (cost per lb of steam)
- Maintenance wise
Since there are two major disadvantages reciprocating pumps present:
- The discontinuity in the discharge flow acting
- They are prone to flow separation at the lowest pressure point in the system.
As we know the fluid is being accelerated and decelerated inside the pump because of reciprocating motion. As designer, applying those principles. When installing such a pump considering the cost and operational performance and maintenance. To overcome such discontinuity flow it is conventional to use pulsation dampeners with some form of diaphragm or bladder for holding the charge, charge is approximately 2/3 of the system pressure. Or instead of installing reciprocating pumps, addressing the problem above screw or rotary pump is preferable in the design because of its assembly, liquid is displaced axially as the screws rotate and mesh it will deliver a definite quantity of liquid with every revolution of the rotors thus resulting to a continues flow. Advantage of rotary pump: Wide range of flows,pressures,liquids and viscosities; High speed capability, allowing driver selection; Low internal velocities; Self priming, with good suction characteristics; High tolerances for entrained air and gases; Minimum churning or foaming; Low mechanical vibration, pulsation- free flow, and quiet operation; Rugged compact design, easy to install; High tolerance to contamination specifically screw pumps. Considering the datum another way is to install a storage tank after reciprocating pump above the level to which a storage tank fluid will continuously flow by means of gravity to receiving equipment.
Therefore in conclusion even when the number of plunger or piston is increased in reciprocating pump there is always a minimal gap or discontinuity to the discharge flow due to its speed and a back and forth stroke inside a single or double acting cylinder commonly called reciprocating motion. And that also causes the flow separation during at lowest pressure point.
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