Pump

Pump and compressor has the same basic. The medium goes to the impeller through the reduction of surface area. By the centrifugal force, the medium will be thrown to the outside. Then, there is a diffuser to increase the area at the outlet and convert the kinetic energy to the static pressure. The important difference between pump and compressor would be the change of gas properties by pressure.

There is always a check valve after the pump not before the pump to protect the pump from the false direction. Before the pump, a pressure loss can result in cavitation.

Basics of pump calculations:

The pressure side of the pump is smaller than the inlet side for the prevention of the cavitation. Th

Required Pressure of Pump: the sum of pressure losses, back pressures and geodetic height.

Velocity: influence the following parameter

1. NPSH in the suction side
2. Energy consumption of the pump
3. Turbulency
4. Corrosion (the higher the velocity, the more corrosion)
5. Sedimentation

• The decision should be made beween lower investion costs and higher energy consumption
• The slower the better with the exception for sedimentation
• The normal suction velocity: 0,5–1 m/s ( It is very important for boiling medium causing cavitation): The boiling medium should have a lower velocity than a subcooled medium. NPSH is important for the velocity.
• The normal velocity on the pressure side: 1,5–3 m/s
• With increasing the flowrate, the pressure loss in the pump will be increased. Sometimes,the pressure loss in the pump will be more than the increased pressure in the pump.

NPSH:

NPSHrequired including 3% cavitation: provided by the pump manufacturer, depends on the following parameters:

1. Instance turbulence as the liquid strikes the impeller
2. Losses created in suction passage
3. Losses ocurred by the liquid passing through the vanes

• Calcuclate required NPSH in case of using water, since the pumps should be started usually with water

NPSHavailable: Absolute Pressure on the surface of the liquid + Elevation Pressure-Pressure Losses- Saturation Pressure-Dynamic Pressure at the Suction Side (coming from Bernoulli equation between two point: surface of the liquid and suction of the pump)

From practical side to prevent the cavitation, wich occurs through a vaccum in the pump due to high velocity and low pressure of impeller in the suction side causing mechanical damage:

NPSH available+0,5/1 meter for pressure loss due to velocity in the suction side>NPSHrequired

If we have a vessel before a pump and the cavitation occurs, the following solutions can be recommended:

• Heat tracing of vessel to increase the pressure over the vapor pressure in winter
• Using Blasenspeicher to remove vapor before entering the pump by increasing the pressure
• Changing the pump
• A boiling medium can be cooled down
• Increasing the level in the vessel prior to the pump
• Reducing the flow rate by having an orifice

Efficiency

1. Volumetric efficiency: actual flow at a given pressure (using a flow meter)/theoretical flow at a given pressure (pump displacement/revolution by driven speed). It is not important for the efficiency calculation.

2. Mechanical efficiency: (rotor/impeller power)/ (shaft power): The most mechanical loss is caused by bearings.

Pump Operating Point: the intersection of the pump curve and the system curve. It comes from a balance between the provided pressure through the pump and the pressure loss through pipelines and fittings.

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The system head will never start from 0. Otherwise, no pump would be required, which is not true. The stystem head at 0 flow is the minimum required pressure provided by the pump.

For every change in the system, this curve should be considered.

If you add a restriction orifice with a flowrate 10 m3/hr to the pump with 50 m3/hr, the pressure loss of system will be higher and the pressure increase in the pump will be lower. It means that the flowrate in the bypass will be reduced due to lower pressure.

Flow turbulency in the pipeline can be specified from Reynolds number using the operating point and cross sectional area of the pipeline.

Pump Calculation

Affinity laws:

The affinity laws (also known as the “Fan Laws” or “Pump Laws”) for pumps/fans are used to express the relationship between variables involved in pump or fan performance (such as head, volumetric flow rate, shaft speed) and power. They apply to pumps and fans.

Buckingham Pi theory is often used in similarity theory to identify the relevant dimensionless groups. The affinity laws are originated by Buckinfham Pi theory.

Dynamic type pumps obey the affinity laws:

1. Capacity varies directly with impeller diameter and speed.

2. Head varies directly with the square of impeller diameter and speed.

3. Horsepower varies directly with the cube of impeller diameter and speed.

Example:

How will be the volume capacity, head, and power of the pump influenced, if the wheel velocity and wheel diameter are being doubled?

1. Head will be 16times higher: The head is calculated as follows:

Head= Density*Acceleration*Height

Acceleration: r(w2) will be 8 times higher

Height will be 2 times higher

Head will be 16 times higher

The affinity laws’ assumptions:

1. The efficiency of pump does not change by different flow rates.
2. The physical properties can be kept constant.

Recommendation:

When considering a larger impeller to expand a pump’s capacity. try to stay within the capacity of the motor. Changing impellers is relatively cheap. but replacing the motor and its associated electrical equipment is expen- sive.

New Pump:

Suction

• Calculation of pressure loss on the suction side of the pump by using a darcy-weisbach formula per length. The equivalent length of valves and fittings should be calculated. (P4)
• The lowest pressure occurs right at the impeller inlet where a sharp pressure dip occurs.
• The suction system is to main- tain the pressure above the vapor pressure at all points.
• Vapor pressure at the operating temperature on the suction side of the pump (P3)
• Pressure of vessel (P1)
• Static pressure due to elevation (P2)

P suction side/NPSHa = P1+P2 -P3-P4

• For studies or initial design it is good to have quick estimates of pump NPSH. Evans discusses the general formula

n = Speed, rpm
Q = Capacity, gpm
C = A constant between 7,000 and 10,000

Evans plotted the relationship with C = 9,000 resulting in the following graph for initial estimates of minimum NPSH required.

• Suction System NPSH with Dissolved Gas: In that case the suction liquid’s vapor pressure is a term in the equation. With dissolved gases, the gas saturation pressure is often much higher than the liquid’s vapor pressure. When mechanical damage is of prime concern, use the Henry rule to estimate the partial pressure of dissolved gas. Considering the vapor pressure of gas can result in a very low NPSH, which is not logical. This is the surest, but most expensive, method. Otheraw’ e, use a lower pressure that will allow no more than 3% by volume of flashed gas in the pump suction.

Discharge

• Calculation of pressure loss on the suction side of the pump by using a darcy-weisbach formula per length. The equivalent length of valves and fittings should be calculated. (P4)
• Pressure of vessel (P1)

P discharge = P1+P4

Hydraulic power of pump: 1.1* (P discharge — P suction)* Volumetric flowrate/(mechanical efficiency of the pump)

Mechanical power of pump: the power at the impeller/the power at the inlet of shaft

• Hydraulic efficiency of a pump depends on the perfomance of impeller and shaft.
• Electrical efficiency is dependent on the motor. If the electrical efficenccy would be too low, the pump could glow. The losses of an electrical machine (see drive) cause the active parts such as the winding (Wicklung) and laminated cores to heat up (see also motor protection). An overload causes an increased power consumption of the electric motors. The permissible limit of heating is determined by the insulating material used (isolator made up of lack)

Shaft torque= power (watt)/(2*3.14*speed/60)

Speed=voltage*number of poles*magnetic field of strength*number of conductors/number of return paths

power(watt)=voltage*current

more poles mean more magnetic fields interacting with stator windings, producing a stronger torque.

The mechanical efficiency is around 90% and the electrical efficiency 98%. If the electrical efficiency is too low, the isolator can not bear a high temperatur. An isolator is used for isolating a circuit a source of power.

Normally, the general efficiency would be reported in the curve by the manufacturer. If the efficiency is low, it should be discussed with the manufacturer to change the motor (increase number of poles, …) or impeller…

• An equation was developed by the author from the pump efficiency curves in the eighth edition of The GPSA Engineering Data Book.’ provided by the M. W. Kellogg Co.

• 10% would be recommended for the overdesign.

In the datasheet of the pump, the overall efficiency would be reported: electrical efficiency (ca. 95%)* mechanical

• Most pumps need minimum flow protection to protect them against shutoff. At shutoff, practically all of a pump’s horsepower turns into heat, which can vaporize the liquid and damage the pump.
• The process engineer must plan for minimum flow pro- visions when making design calculations. For preliminary work, approximate the required minimum flow by assum- ing all the horsepower at blocked-in conditions turns into heat. Then, provide enough minimum flow to carry away this heat at a 15°F rise in the minimum flow stream’s temperature.

Existing Pump

The pump curve should be used for evaluating the performance of the pump.

Pumps in Serial — Head Added

When two (or more) pumps are arranged in serial their resulting pump performance curve is obtained by adding their heads at the same flow rate as indicated in the figure below.

Pumps in Parallel — Flow Rate Added

When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding the pumps flow rates at the same head as indicated in the figure below.

Pump Classifications

2. Flow:

1. Radial flow: low flow and high head

2. Axial flow: high flow and low head

3. Mixed flow: high flow and high head

The pump needs a minimum flowrate, which can be gained through the following methods:

1. Frequency Converter: It can be used to control the motor speed and cause the pump curve to shift. It is not common to use frequency converter in industry.
2. Using a vessel to keep fluid for ( Infrastructure would be very big)
3. Change the position of pump to get required NPSH
4. Having a bypass with restriction orifice on the pressure side of the pump
5. Having a valve on the pressure side of the pump

A normal frequency of the motor is around 3000 rpm (50rps: 50Hz)

1. Impeller:

• The thinner the impeller is, the better the efficiency would be
• The more closed, the better the efficiency would be

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1. Closed: applied in large pumps and not recommendable for solids
2. Open: applied in small pumps and good for solids or dirty medium, has a lower efficiency than closed
3. Semi-Open: a terrible efficiency

Multistage pumps: The head of a single-stage centrifugal pump is essentially determined by the design of the impeller and the peripheral speed. If the speed cannot be further increased due to other boundary conditions and if the increase in the impeller diameter leads to very low specific speeds and thus to uneconomic efficiencies, the delivery head can be increased economically by connecting several stages in series (see also series operation).

Side channel pump: They contain multiple impellers similar to a multistage pump, however the design of the impellers are slightly different being more like a multistage peripheral pump. The side channel pump is a specialist in particular areas of application that occupy the niche between centrifugal pumps and positive displacement pumps. These pumps offer many benefits: They are self-priming, not sensitive to cavitation, they can easily handle fluids with high gas content, and they are suitable for pressure-dependent circulation regulation. Generally, side channel pumps are especially good for “low flow — high head” applications.

Types of pumps:

1. Positive Displacement

• Pulsation dampers are used after and before pump systems to reduce the pulsations of discontinuous pumps, e.g. B. reciprocating pumps and diaphragm pumps, and to achieve a constant flow rate and quantity. It will be filled with a gas to a defined pressure.

DeltaV: Volume of the liquid introduced to the damper

V2: Final gas volume

V: Spare volume for hammering (0,2V0)

V0: Volume of the damper

V0=V2+DeltaV+V

V0=(V2+DeltaV)/0.8

• Flowrate remains constant with a change in pressure
• Efficiency is less affected by pressure. Therefore, the pressure at the pressure side of pump must be controlled. Otherwise, the uncontrolled high pressure can damage the fittings of pipeline. They have normally a suction valve and a pressure valve for the regulation of flow in to the chamber.
• The difference pressure is dependent on the process. The volumetric flow rate would be the result of area of the piston * rotation speed
• It can provide high pressures with low flow rates
• It is usually used for small dosing amount

Problem: high pressure should be controlled. There are three methods:

• Variodeckel: to send the medium from pressure side to the suction side
• Manometer: to use a manometer after the pump. In case of a high pressure, the motor will be shutted off.
• Bypass: Using a bypass from a suction side to the safety valve.

I. Diaphragm Pump (Membranpumpe): that uses a combination of the reciprocating action of a rubber diaphragm and suitable valves on either side of the diaphragm to pump a fluid. The diaphragm will be pressurized through a chamber containing fluid such as oil.

• This type of pump needs no sealing

II. Gear Pump (Zahnradpumpe): good for viscous medium

V. Drehkolbenpumpe: It is like a gear pump

Börger provides this type of pump with a maximum pressure of 16 bar and a capacity of 1600 m3/h.

III. Piston Pump (Kolbenpumpe)

The axial reciprocating motion of the pistons is obtained by a swash plate (Taumler) that is either fixed or variable in its degree of angle.

IV. Progressive cavity pumpe (Exzenterschenkenpumpe): It is highly recommended for viscous and abrasive medium having solid particles, since there is no valve which will be plugged. But, there are check valves in the other types of reciprocating pumps.

For achieving a high pressure, the pump can be used in multi-stage.

V. Schlauchpumpe: It is in our body and used mostly in the food industry.

2. Centrifugal Pump:

conversion of rotational energy through impellers typically coming from an electric motor or steam turbine to hydrodynamic energy of fluid. There is a centrifugal force in pump, which results in a static pressure (Force/Area).

• Flow rate varies with a change in pressure
• Efficiency peaks at a specific pressure
• It can provide low pressures with high flow rates
• The caused pressure difference is always constant as long as the impeller and the rotation speed are constant:(V2/r)

Problem: Bearing and Shaft Sealing

Dry running of sealing:

The most pumps use the liquid to be pumped in bearing and sealing for flushing and cooling. The following problems can occur:

• Blockage due to particles in the liquid
• Lubrication film interruption due to boiling medium
• Evaporation due to warming caused by a closed gate valve or cavitation

Compensation solution:

• Using filter or pressure difference measurement to avoid entering particles
• Using another medium (Externe Spül/Sperrmedium) for flushing and cooling
• Using alarm signal by a minumum level
• Tempearture measurement

Pump Leakage:

There is normally leakage, where shaft leaves casing. Pump leakage can result in following items:

1. Pollution: According to TA-Luft, every should have a mechanical seal to prevent the emission of pollutants to air.
2. Corrosion
3. Product loss
4. Reducing efficiency

To avoid the pump leakage:

1. Compression packing (Stopfbuchse, Packungsringe)cannot be 100% leak free. Hence, TA-Luft would be problematic for hazardous medium. The contact between the packing and shaft can cause heating and damage.

The sealing system should mostly fullfil TA-Luft, air pollution regulation in oil and gas industry due to hazardous fluids. The following methods would be recommended regarding TA-Luft

If there is not enough sealing, the compression packing should be exchanged.

2. Mechanical seal

• Double mechanical seal: using API plan to control the seal environment, The problems with this type is sealing: lots of sensors, using medium (Kühl and Sperrmedium), being changed every 1–3 years , the pollution of fluid with Sperrmedium. If there is a leakage, the locking medium (sperrmedium), white oil (Weißöl), will leak to the process, since the pressure of the sperrmedium is higher than the process medium.The sealing contains an oil vessel, a pump, a cooler, a sieve, and a regulating valve.
• White oil is obtained by refining (refining / cleaning) paraffin oil. … This white oil is then colorless and odorless, which is used usually as a locking medium.
• Hydrodynamic seal: uses a dynamic rotor with grooves that act as a pump and create an air film that the opposing sealing surface will ride on. As long as, the liquid goes into the pump, the sealing runs smoothly. For shutdowns, it needs another sealing method such as Lippendichtung.
• Labyrinth seal: using high pressure air or Nitrogen for Ex-Zone
• Electromagnetic cluth (Magnetkupplung): operate via an electric actuation, but transmit torque mechanically. When the clutch is required to actuate, voltage/current is applied to the clutch coil. The coil becomes an electromagnet and produces magnetic lines of flux. This flux is then transferred through the small air gap between the field and the rotor. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop that attracts the armature turning around to produce movement. The armature is pulled against the rotor and a frictional force is applied at contact. IT is good for clea
• Canned motor pump (Spaltrohrmotorpumpe): is an integral, compact unit without shaft seal. This type of pump combines the rotating part of the pump hydraulics with the rotating part of the motor on a common shaft. The pump shaft is completely enclosed by a tightly welded rotor lining that separates the rotor assembly from the stator of the drive motor. This design principle eliminates the need for shaft seals (e.g. mechanical seals or shaft bushings) and prevents leakage. Therefore, canned motor pumps are also called ‘sealless’ or ‘hermetically sealed’. It can be used instead of double mechanical seal (DGRD)

To choose the type of pump, the following data are required:

1. Flowrate with composition including chemicals
2. Physical properties: viscosity, temperature and pressure
3. NPSH
4. Ex-Zone (TA-Luft)
5. Operating time

It is highly recommended to use centrifugal pump than positive displacement due to continous flow resulting in no pulsation (Druckschläge). Hence, the following parameter would be suggested:

• Low velocity
• No fast closing of valve
• Using frequency convertor

Centrifugal pump: number of revolution, impeller type, impeller efficiency, bearing and medium, monitoring, guarantee period

Positive displacement pump: number of revolution (specially for gear pump), overpressure, heat tracing for dead spaces, friction force

Self-regulating pump: It is highly recommended for batch applications, slop system, emptying of residue,…, where there is no sufficient NPSH. Through an equalizing gas line (yelow one), cavitation would be prevented for different flow rates without using a frequency converter or even 0 flow rate.

Key Parameters for choosing an appropriate pump:

1. Accquistion cost
2. Reconstructing cost
3. Maintenance cost
4. Utility cost
5. Availabilty factor

Before staring pump, make sure that:

1. Minimum volumetric flow can be provided (IBN)
2. The pump construction materials can tolerate the chemicals (IBN)
3. Cavitation should be checked (IBN)
4. Shaft seal, allows the rotating shaft to enter the ‘wet’ area of the pump without leaking fluids, is correctly installed (IBN)
5. Bearing frame, for keeping the shaft in right alignment, is properly filled with oil. There is an oil bottle, from which oil flows into the bearing frame. The oil losses will be compensated through the visual inspection of oil level in this oil bottle.(IBN)

6. The required meshes for strainer, dependent on the commissioning activity or normal operation, should be provided. The inappropriate mesh size causes the accumulation of dirt in the suction side of pump resulting in high pressure loss and cavitation.

7. All connected signals such as pressure valve and controlling valve should be checked

8. The direction of motor should be checked, which should not be coupled.To prevent a back-flow into the pump, there is a check valve on the pressure side of pump due to enough pressure. If there is a by-pass line to the pump to avoid freezing at very low ambient temperatures, a restriction orifice should be provided to restrict the amount of back-flow. If the direction is installed incorrectly, the electricity line should be exchanged.

9. Coupling guard is installed (Cold alignment)

10. Vent: through opening the valve on the pressure side and runing the pump for multiple times

11. The centrifugal pump shall be started with discharge valve completely colsed and opens this valve gradually until the required flow rate is reached,while positive displacement pump should be started with discharge valve completely opened due to overpressure.

12. If there are two pumps in parallel and the second pump should be started. It should have the operating temperature through the restriction orifice in a cycle before it is connected to the running pump.

During operation, the following checks are required:

1. Check suction pressure
2. Check oil level in the mentioned oil bottle (46 cSt at 40 C)
3. Check bearing temperature for a good lubrication (e.g. not exceed the ambient temperature by 40 C with a maximum of 80C)
4. Check shaft seal performance through leakages
5. Watch for any vibration
6. For high temperature application, the temperature should be stabilized
7. Drain the oil after 200 operating hours or once a year
8. In order to prevent damage due to frost, the pumps should be operated alernatively or a bypass has to be provided on the pressure side of the pump with an restriction orifice to maintain a sufficient back flow through the pump

• If there is air in the pump, the pump can not be runned. For small shut downs, the pump should be full of medium for a good performance.

Failure Reasons

1. Cavitation can cause vibration: lower flowrate (lower pressure loss) and higher suction pressure can imporove vibrations problems.
2. Gas part and solid
3. Inlet conditions
4. Vibration coming from overworked motor. When the basin fills up faster than the motor can pump it out water eventually backs up and causes to motor to overheat, vibrate and eventually flood. Vibration can be caused due to a false alignement.
5. False alignment
6. False Sealing
7. Dry running problem
8. Instrument problem
9. Incorrect operation

It is difficult to evaluate the permonce of each component of pump system:

Pump: Filter (pressure difference measurement/particle remover)+ first blocking (gate valve/schieber)+pump ( sealing, bearing, dry running,..)+ check valve + pipeline (bend, flange, ..)

An important suggestion: the fewer the component (signal, sensor,…), the better the operation

Pump Protection:

1. Using filter before the pump
2. Dry running signal ( minimum level in the vessel before the pump)
3. Temperature measurement in the pipeline and pump)
4. Acoustic sensor
5. Minimum flowrate (Bypass)
6. Heat tracing or cooling
7. Frequency convertor
8. Signals for the sealing system (level, temperature, pressure and flowrate)

An important suggestion: counsulting with manufacturer

Every pump has a dry running protection (Trockenlaufschutz), temperature protection (Temperaturabsicherung, Wirklungstemperatur des E-Motors), electricity protection (Elektrische Sicherung).