Centrifugal pumps valves

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Impervious graphite centrifugal pumps, pipe fittings, and valves were developed because most chemical processes require the movement of Hquids. Graphite pipe and fittings in sizes ranging from 25 to 635 mm ID are used to convey corrosive fluids. 

Unhke the control valve, the centrifugal pump has poor or nonexistent shutoff capabihty. A flow check valve or an automated on/off valve may be reqiiired to achieve shutoff reqiiirements. This requirement may be met by automating an existing isolation valve in retrofit apphcations. 

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

The pumps can be either centrifugal or positive displacement types. The centrifugal pumps should have a head curve continuously rising toward the shut-off point. The standby pump should be piped into the system in a manner that permits checking of the pump while the main pump is in operation. To achieve this, a restriction orifice is required with a test bleeder valve piped to the return oil line or the reservoir. 

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Pump and Downstream Equipment - A PR valve is required for a pump when the shutoff pressure of the pump is greater than the design pressure of the discharge piping, downstream equipment, or pump casing. Positive displacement pumps normally require such protection, while in most cases centrifugal pumps do not. 

Positive pumps are normally fitted with relief valves. These are not usually fitted to centrifugal pumps unless the process material is likely to explode if it gets too hot. As an alternative to a relief valve, such pumps may be fitted with a high-temperature trip. This isolates the power supply. Or a kick-back, a small-diameter line (or a line with a restrietion orifice plate) leading from the delivery line baek to the suction vessel, may be used. The line or orifice plate is sized so that it will pass just enough liquid to prevent the pump from overheating. Small-diameter tines are better than restriction orifice plates as they are less easily removed. 

Positive displacement pumps are self-priming by their normal operating action and are designed to cope with mnning dry. Centrifugal pumps are not inherently selfpriming, and need to be provided with assistance in the form of ejectors, as in Figure 32.56 or dry vacuum pump systems. In these systems, a dry vacuum pump allows air to be drawn out and where an automatic valve prevents... 

This procedure provides detailed instructions on how to repack centrifugal pump packed stuffing boxes or glands. The methodology described here is applicable to other gland sealed units such as valves and reciprocating machinery. 

Rupture disk, 455 Safety relief valves, 454, 467, 481 Specifications, centrifugal pumps, 209 Spray nozzle particle size, 225 Standards and Codes, 31, 32, 33 Static electricity, 536 Static mixing, 332 Applications, 336 Calculations, 337, 338 Materials of construction, 337 Principles of operation, 335 Type of equipment 334-338... 

Most centrifugal pumps are controlled by throttling the flow with a valve on the pump discharge, see Section 5.8.3. This varies the dynamic pressure loss, and so the position of the operating point on the pump characteristic curve. 

A well point system consists of several individual well points spaced at 0.6 m to 1.8 m intervals along a specified alignment. A well point is a well screen (length 0.5 to 1.0 m) with a conical steel drive point at bottom. Individual well points are attached to a riser pipe (diameter 2.5 to 7.5 cm) and connected to a header pipe (diameter 15 to 20 cm). At the midpoint, the header pipe is connected to a centrifugal pump. As yield at different well points may vary, a valve at the top of each riser pipe is used to control the drawdown so that the screen bottom is exposed. The pump provides 6 to 7.5 m of suction, but friction losses reduce the effective suction to 4.5 to 5.4 m. 

Toluene is to be pumped between two vessels using a centrifugal pump with a flowrate of 30t h 1. The pipe diameter is 80 mm (internal diameter 77.93 mm). The pipeline is 35 m long, with 4 isolation valves (plug cock), a check valve and 5 bends. The discharge tank is 3 m in elevation above the feed tank. The density of toluene is 778 kgrn 3 and viscosity of 0.251 x 10 3 N-s-rn 2. 

Effect of throttling the discharge valve on the operating point of a centrifugal pump... 

The piping and valves may be arranged to enable two centrifugal pumps to be operated either in series or in parallel. For two identical pumps, series operation gives a total head of 2Ah at a capacity Q and parallel operation gives a capacity of 2Q at a total head Ah. The efficiency of either the series or parallel combination is practically the same as for a single pump. 

Lantern rings for pumps, bellow-type seals for centrifugal pumps hydraulic, pneumatic, earthmoving equipment cylinder valves for O2, CO2, refined gases, chlorine, ammonia... 

The process engineer s job is to size both the centrifugal pump and the control valve. The bigger the control valve, the less pressure drop it will take. This means a lower-head pump can be used and energy costs will be tower... 

The higher flow rate might also reduce the head that the centrifugal pump produces if we are out on the pump curve where head is dropping rapidly with throughput. For simplicity, let us assume that the pump curve is flat. This means that the total pressure drop across the heat exchanger and the control valve is constant. Therefore, the pressure drop over the control valve must decrease as the the pressure drop over the heat exchanger increases. 

The designer must specify the maximum flow rate that is required under the worst conditions and the minimum flow rate that is required. Then the valve flow equations for the maximum and minimum conditions give two equations and two unknowns the pressure head of the centrifugal pump APj> and the control valve size C . 

J4. Reactant liquid is pumped into a batch reactor at a variable rate. The reactor pressure also varies during the batch cycle. Specify the control valve size and the centrifugal pump head required. Assume a flat pump curve. 

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