Water Hammer On Centrifugal Pump

Pumping problem that often facing is water hammer, and issue of wrong pump and pipe designer that sometime can’t be handle by new operators. Water hammer often happened on a big centrifugal pump when wrong operate.

How it can happen?
Water hammer practically cannot detect if the increasing pressure accelerated or retarded pump by pump condition and valve changes can be normalize automatically. Usually this pressure changes just small, the rate of changes also gradual. If the pressure changes may ten times bar, and the forces on pump support can many tones till exceeding the support limitation, the pipe can be rupture or damage. Other case of lower level water hammer can cause chronic deterioration, and leading to the gasket leaks over time.

Water hammer can happen if the valve open or closed to fast, this action can cause pressure changes too quickly. The big pressure changes can cause the force to the pipe or to the pipe support out of balance. The force like this can shift the pipe even move from the pipe support.

How to avoid water hammer?
Water hammer can be avoided by many kind depend on each situation’s and circumstance. The following may be some conclusion as your consideration to apply.

• Eliminating of problem source like from vibrating pressure of relief valve, fast emergency shutdown valve closure, change of the automatic close time like on the butterfly valve and many others.
• Reduce the pumping velocity, this can be modified on the pump like to install expansion pipe to reduce pressure.
• Install surge tanks
• Use surge alleviators that commonly just fit for positive displacement pump.
• Use air inlet valves.
• Create a by pass pipe that passing the valve when closed.

Centrifugal Pump With Magnetic Bearing

Magnetic bearing maintain the rotor of a pump in suspension through the force of attraction of a magnetic circuit. Thus although they bear up to weight and hydraulic loads of the impellers and the shaft, they are not really bearings in the traditional sense of the rotating and stationary surfaces bearing on one another. The supporting magnet circuit for each bearing includes stationary magnets in a stator surrounding the shaft, a laminated rotor that on the shaft, and the shaft itself. The stator consists of electromagnet in the traditional heteropolar design, and if a homopolar design is employed, permanent magnets can be added. Sensors monitor the position of the shaft and signal a controller to adjust the magnetic loads to keep the shaft to within about 0.001 in (26 μm) of the desired position.

Magnetic bearings are found in small, high speed turbomachinery such as high speed, multistage, axial flow turbomelecular vacuum pumps. They were introduced into large turbomachinery in the nearly 1980s, mainly in gas compressors and truboexpanders. Their use and acceptance has grown slowly but steady since then. Pump applications of a signings can provide a technically sound bearing with maintenance and operating advantages, including zero wear. However, due to the technical complexity of magnetic bearing systems, the economics of scale associated with production quantities are required to make these systems affordable.

Two representative magnetic bearing equipped pumps are summarize in the table below. One is a multistage boiler feedwater pump and the other a single stage double suction hydrocarbon process pump. The multistage pump was retrofitted with magnetic bearings, together with another identical pump that still contains the oil lubricated bearing, both installed in an electric generating station. The magnetic bearing pump is not encumbered with the usual complexity of a bearing lubrication system.

Parameter	Multistage Pump	Single Stage Pump
Power, hp (MW)	610 (0.46)	800 (0.6)
Rated speed, rpm	3580	1780
Shaft weight, lb (kN)	800 (3.6)	Thrust end: 930 (4.1) Drive end: 1415 (6.3)
Thrust bearing design load, lb (kN)	4000 (17.8)	4000 (17.8)
Number of Stages	8	1

Typical parts in a centrifugal pump

A centrifugal pump is a relatively simple pump. Design, types and numbers of parts vary depending on centrifugal pump brand, type and configuration.

Typical main pump parts:

Casing/backplate:

• Contains impeller where fluid is transferred from inlet to outlet.
• Includes inlet and outlet ports.
• Typically flexible port orientation.
• Typically fitted to an adapter

Shaft:

• Rotates impeller which is fixed to it.
• Is fixed to the motor and rotates with it.

Impeller:

• Transfers fluid from inlet to outlet with increased capacity and pressure
• Is fixed on the shaft and rotates with it.
• Typical types are open, semi-open or closed.

Shaft seal:

• Seals between rotating shaft and stationary casing.
• Typically a mechanical seal, external or internal.
• Typically available as single, single flushed and double flushed seal.

Adapter:

• Fixes pump casing to the motor.

Motor:

• Rotates shaft (impeller) which is fixed to it.
• Typically a 3-phase electrical motor.
• Typically available for various electrical site supplies (voltage and frequency).
• Typically available in various protection classes (flameproof etc.).

Other parts:

• Seals, motor cover, seal flushing, coupling/base (base-mounted pump).

Typical materials:

• Steel parts of 316L or 304 stainless steel.
• Elastomers of NBR, EPDM, FPM, PTFE.

The Principle of a centrifugal pump

The centrifugal pump transfers fluid at a certain capacity from one point to another in a process. The pump builds up fluid pressure to overcome losses in the process. Capacity and pressure are created by the rotating impeller inside the pump casing.

 

General principle:

·         Fluid enters the pump casing and impeller center and is forced into a circular movement by the impeller vanes and the centrifugal force. The fluid thus leaves the casing with increased pressure and velocity.

·         Typically suitable for low viscous, non-particulate and non-aerated fluids such as beer, CIP, cream, juice, milk, soft drinks, water etc.

 

Single-stage principle:

The fluid inlet, the built-up of velocity and pressure and the fluid outlet all happens in one stage (one casing and one impeller).

 

Multi-stage principle:

·         Fluid enters the pump casing and impeller center, and fluid pressure and velocity are built up in the first stage (casing and impeller) similar to the single-stage pump.

·         Fluid with increased pressure and velocity is directed to the second stage (casing and impeller), where the fluid pressure and velocity is further increased.

·         The result is a pressure increase (boost) in each stage, where the total pressure increase depends on the number of stages in the pump.

·         Typically available with 2-4 stages.

 

Priming of a centrifugal pump:

·         The pump casing should always be filled with fluid before starting the pump to ensure correct operation.

·         The pump can operate with a positive inlet pressure (flooded inlet) or with a negative inlet pressure (suction lift).

·         For suction lift, fluid can remain in the pump casing by using a non-return valve in the suction line.

 

Basic of Centrifugal Pump

A centrifugal pump is typically the most common sanitary pump type used in sanitary processes. Benefots include a relatively low purchase cost, wide selection, simple design and easy maintenance.

The various aspects of centrifugal pumps are very important to consider when dealing with flow technology and flow equipment. Understanding the aspects of centrifugal pumps makes it easier to select correct pumps, optimize processes and minimize costs.

Process with centrifugal pump (principle)

On the process itself centrifugal pump can arrange in many type, the example of centrifugal pump arrangement on the process system can see on the picture bellow:

Centrifugal Pump Type

Centrifugal pump design can see the parts assembly as follows:

Centrifugal Pump Design

Examples of centrifugal pumps

1. A centrifugal pump is used in processes with non-viscous and nonparticulate fluids, e.g. beer, CIP, cream, milk, soft drink and purified water.
2. There are typically many types of centrifugal pumps available for various types of applications.
3. The main parts of a centrifugal pump are motor, shaft, adapter, shaft seal, impeller, casing and seals.
4. A centrifugal pump is typically selected from a pump curve or a pump selection program.
   

Centrifugal Pump Characteristic

It is important to note that at any fixed speed the pump well operate along this certain curve and no other point for instance on the curve below. On pump with variable speed driver such as steam turbine, it is possible to change the characteristic curve. It is important to remember that the head produced will be the same for any clean liquid of the same viscosity. The pressure rise, however, will in proportion to the specific gravity. Viscosities of less than 50 kPa's (50 cP) do not affect the head materially.

Single Stage Centrifugal Pump
This pump are available to capacities up to and over 1.136 x 104 m3/h (50,000 gal/min) for heads (pressures) up to 488 m (1600 ft). They are available in a variety of design for particular services.

Process Pumps
This term is usually applied to single stage pedestal mounted units with single suction overhung impellers and with a single packing bos. These pumps are ruggedly designed for ease in arrangements, and are built especially to handle corrosive or other wise difficult to handle liquids.

Specially but not exclusively for the chemical industry, most pump manufactures now build to national standards horizontal and vertical process pumps. American National Standards Institute (ANSI) Standard B73 1 - 1977 and B73 2 - 1975 apply to be horizontal and vertical in line pumps respectively.

The horizontal pumps are available in line pumps for capacities up to 900 m3/h (4000 gal/min), the vertical in line pumps, for capacities up to 320 m3/h (1400 gal/min). Both horizontal and vertical in lime pumps are available for heads up to 120 m (400 ft). The intent of each ANSI specification in that pumps from all vendors for a given nominal capacity and total dynamic head at a given rotative speed shall be dimensionally interchangeable with respect to mounting, size, and location of suction and discharge nozzles, input shaft, base plate, and foundation bolts.

Centrifugal Pump

The centrifugal pump is the type most widely used in the chemical industry for transferring liquids of all types, raw material, material in manufacture, and finished product, as well as for general services of water supply, boiler feed, condenser circulation, condenser return, etc. This pump are available through fast range of sizes in capacities from 0.5 m3/hour to 20,000 m3/hr (2 gal/min to 100,000 gal/min) and for discharge head (pressure) from a few meters to approximately 48 MPa. The size and type best suited to particular applications can be determined only by an engineering study of the problem.

The primary advantages of a centrifugal pump are simplicity, low first cost, uniform (nonpulsating) flow, small flow space, low maintenance expense, quite operation, and adaptability for use with a motor or turbine drive.

Centrifugal pump in a simplest form, consists of an impeller rotating within a casing. The impeller consists of a number of blades, either open or shrouded, mounted on a shaft that projects outside the casing. Its axis of rotation may be either horizontal or vertical, to suit the work to be done. Closed-type, or shrouded, impellers are generally the most efficient. Open or semi open type impellers are used for viscose liquids or liquids containing solid materials and on many small pumps for general service. Impeller may be of the single suction or double suction type, single if the liquid enter from one side, double if it enter from both side.