As we all know that pumps are the essential part of any pumping system where the movement of liquid is done by mechanical actions. In this blog, we are going to discuss mainly the principle of working, various pump parts, operative challenges and about the future of centrifugal pumps which is widely used for our domestic and marine applications.
One fact that must always be remembered: A pump does not create pressure, it only provides flow. Pressure is a just an indication of the amount of resistance to flow
Nowadays for marine application mainly two types of pumps are used.
- Centrifugal pump
- Positive displacement pump
Centrifugal pump construction: It consists of a stationary pump casing and an impeller mounted on a rotating shaft. The impeller contains rotating vanes that impart a radial and rotary motion to the liquid when it is rotating at a high speed. With the help of centrifugal action, water tends to move out to the periphery of the rotating impeller and causes a drop in pressure at the center and hence liquid flows from a suction line to the eye.
Centrifugal pump working.
The pump casing guides the liquid from the suction connection to the center, or eye, of the impeller. The vanes of the rotating impeller impart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pump casing where it is collected in the outer part of the pump casing called the volute. The volute collects the liquid discharged from the impeller at high velocity and gradually causes a reduction in fluid velocity by increasing the flow area, converting the velocity head to a static pressure head. The fluid is then discharged from the pump through the discharge connection.
Centrifugal Pump Components
It rotates the liquid trapped in the pump casing and gives kinetic energy. This kinetic energy is further converted into pressure energy with the help of diffuser and volute casing.
It can be divided into two types in reference to the suction
- Single-suction impeller allows liquid to enter the center of the blades from only one direction.
- Double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously.
Based on the direction of flow in reference to the axis of rotation
- Radial flow
- Axial flow
- Mixed flow
Some centrifugal pumps contain diffusers. A diffuser is a set of stationary vanes that surround the impeller. The purpose of the diffuser is to increase the efficiency of the centrifugal pump by allowing a more gradual expansion and less turbulent area for the liquid to reduce in velocity. The diffuser vanes are designed in a manner that the liquid exiting the impeller will encounter an ever-increasing flow area as it passes through the diffuser. This increase in flow area causes a reduction in flow velocity, converting kinetic energy into flow pressure.
Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow a small running clearance between the impeller and pump casing without causing wear of the actual impeller or pump casing material. To maximize the efficiency of a centrifugal pump, it is necessary to minimize the amount of liquid leaking through this clearance from the high pressure or discharge side of the pump back to the low pressure or suction side. If the clearance becomes too large, internal recirculation takes place from high-pressure discharge side to low-pressure suction side of the pump, resulting in heat generation due to the churning of liquid and vibration problems with a drop in pumping efficiency. Most manufacturers advise to check the wear ring clearance and replace the rings when this clearance crosses the allowable limit.
In almost all centrifugal pumps, the rotating shaft that drives the impeller penetrates the pressure boundary of the pump casing. It is important that the pump is designed properly to control the amount of liquid that leaks along the shaft at the point that the shaft penetrates the pump casing. There are many different methods of sealing the shaft penetration of the pump casing. Factors considered when choosing a method include the pressure and temperature of the fluid being pumped, the size of the pump, and the chemical and physical characteristics of the fluid being pumped.
One of the simplest types of shaft seal is the stuffing box. The stuffing box is a cylindrical space in the pump casing surrounding the shaft. Rings of packing material are placed in this space. Packing is material in the form of rings or strands that is placed in the stuffing box to form a seal to control the rate of leakage along the shaft. The packing rings are held in place by a gland. The gland is, in turn, held in place by studs with adjusting nuts. As the adjusting nuts are tightened, they move the gland in and compress the packing. This axial compression causes the packing to expand radially, forming a tight seal between the rotating shaft and the inside wall of the stuffing box.
The high speed rotation of the shaft generates a significant amount of heat as it rubs against the packing rings. If no lubrication and cooling are provided to the packing, the temperature of the packing increases to the point where damage occurs to the packing, the pump shaft, and possibly nearby pump bearings. Stuffing boxes are normally designed to allow a small amount of controlled leakage along the shaft to provide lubrication and cooling to the packing. The leakage rate can be adjusted by tightening and loosening the packing gland.
A lantern ring is a perforated hollow ring located near the center of the packing box that receives relatively cool, clean liquid from either the discharge of the pump or from an external source and distributes the liquid uniformly around the shaft to provide lubrication and cooling. The fluid entering the lantern ring can cool the shaft and packing, lubricate the packing, or seal the joint between the shaft and packing against leakage of air into the pump in the event the pump suction pressure is less than that of the atmosphere.
In some situations, packing material is not adequate for sealing the shaft. One common alternative method for sealing the shaft is with mechanical seals. Mechanical seals consist of two basic parts, a rotating element attached to the pump shaft and a stationary element attached to the pump casing. Each of these elements has a highly polished sealing surface. The polished faces of the rotating and stationary elements come into contact with each other to form a seal that prevents leakage along the shaft.
Cavitation in centrifugal pumps
Cavitation is a thermodynamic change of state of liquid into vapor due to localize very low-pressure zones created by mechanical and fluid dynamics. These vapors collapsed very fast as they reach to the high-pressure zones and creates a void or low-pressure area. When bubbles burst, the fluid surrounding this void rushes in to fill the space. The implosion of those bubbles triggers intense shockwaves, causing erosion and premature wear of the respective area, like impeller tips, and pump housing.
Cavitation may lead to:
- Performance loss (head drop)
- Material damage (cavitation erosion)
- Vibrations, where Excessive pump vibration Damage to pump impeller, bearings, wearing rings, and seals
- Abnormal Noise
- Vapor lock (if suction pressure drops below break-off value)
- Load fluctuation across the pump prime mover.
The consequences of prolonged conditions of cavitation and low flow operation can be disastrous for both the pump and the process. Such failures in hydrocarbon pumping services have often caused damaging fires resulting in loss of the machine, production, and worst of all human life.
There are three indications that a centrifugal pump is cavitating.
- Noise Fluctuating
- Discharge pressure and Liquid flow Fluctuation.
- Hunting of pump motor current
Steps that can be taken to stop pump cavitation include:
- Increase the pressure head at the suction of the pump.
- Reduce the temperature of the liquid being pumped in case of highly volatile liquid.
- Reduce head losses in the pump suction piping.
- Reduce the flow rate through the pump by throttling the discharge valve.
- Reduce the speed of the pump impeller.
Capacity of a centrifugal pump depends on various factors which include:
- Process liquid characteristics i.e. density, viscosity
- Size of inlet and outlet sections of the pump.
- Diameter of Impeller and rotational speed.
- Size and shape of cavities between the vanes
- Pump suction and discharge temperature and pressure conditions
To measure a centrifugal pump’s energy, we use head instead of pressure because the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. Since any given centrifugal pump can move a lot of different fluids, with different specific gravities, it is simpler to discuss the pump’s head and forget about the pressure.
Net Positive Suction Head:
Absolute suction pressure head(Hsuc): The pressure acting on the surface of the liquid on the suction side is an Absolute suction pressure head. In an open atmosphere, it is same as atmospheric pressure but in case of the closed tank, it may vary as per tank pressure condition.
Static Suction Head (Hs): Head resulting from a height of the liquid surface relative to the pump centerline. If the liquid level is above pump centerline, Hs is positive. If the liquid level is below pump centerline, Hs is negative. Negative Hs condition is commonly denoted as a “suction lift” condition
Friction Head (Hf): As we know that if there is any relative motion friction is always in the picture at the contact surfaces. Similarly, when liquid is moving inside the piping, friction always opposing the flow. The head required to overcome the resistance to flow in the pipe and fittings is known as friction head. It depends upon the size, condition, and type of pipe, number, and type of pipe fittings, flow rate, and nature of the liquid.
Vapor Pressure Head (Hvap): Vapor pressure is the pressure at which a liquid and its vapor co-exist in equilibrium at a given temperature. When the vapor pressure is converted to head, it is referred to as vapor pressure head, Hvp. The value of Hvp of a liquid increases with the rising temperature and in effect, opposes the pressure on the liquid surface, the positive force that tends to cause liquid flow into the pump suction i.e. it reduces the suction pressure head.
Velocity Head (Hv)-The amount of energy required to bring a fluid from standstill to its velocity is known as velocity head. The velocity head varies indirectly to the diameter of suction. However, it can be a large factor and must be considered in low head systems.
Net positive suction head available(NPSHa): The algebraic sum of the above suction side pressure head components represents the total pressure available at the pump suction.
Net positive suction head required(NPSHr)this value expresses the minimum absolute pressure that must be acting on a liquid as it enters the pump impeller to avoid excessive cavitation and degradation of pump performance. This value is given by the pump manufacturer for the efficient operating range of a centrifugal pump.
So now we can conclude that NPSHa is a function of your pumping system and must be calculated, whereas NPSHr is a function of the pump and must be provided by the pump manufacturer. NPSHA MUST be greater than NPSHr for the pump system to operate without cavitating.
Consider images given below for the calculation of NPSHain various condition. And discharge condition is also calculated, that is only to show the total efforts to transfer liquid from the suction tank to the discharge tank.
Discharge Condition can be expressed as:
And the efforts by any pump for a given system can be calculated as the total head loss incurred in the system.
Latest technology developments of centrifugal pumps and Pump control system.
As we know that the centrifugal pumps are having few limitations along with some operational problems and continuous technological developments are going on for better performance. The main area of concern for pumps can be divided into two parts:
Material technology and Parts design optimization
Competitiveness in the market has pushed centrifugal pump technology development in the area of performance and overall design and operational cost reduction. During operation, pumps specific areas are subject to corrosion, erosion, and fatigue. From a design perspective, an appropriate material selection is necessary for satisfactory machine life. Nowadays metallurgical industries have better processes control over the chemistry of materials. This evolution of material technology has developed in such a way that it can promote the pump parts for better resistance to harsh conditions.
- The quality of pump shaft steel has been significantly improved to withstand with more stresses, vibration, and fatigue.
- New heat treatment processes such as plasma or laser coatings for impeller and sleeve surface hardening allow for better wear resistance.
- With the help of modern computer technology, a better understanding of the stress field inside pump components can be achieved and parts material selection can be made accordingly. Increasing computational power and the availability of reliable codes allowed consistent improvements to the centrifugal pump design process and the integration of original equipment manufacturer experience with reliable simulations and predictions
- Modern design takes advantage of the availability of software tools that allow delicate valuations in mechanical design points such as bearing selection, balancing position and sleeve clearances sizing. These elements and the boundary design operative conditions contribute to the final stability of the rotor.
- New generation flexible pump coupling, which can withstand with a larger degree of misalignment and reduce vibration. With reduced vibration pump parts and bearing life will improve further.
2-Operational control system and process monitoring.
- In the earlier days, pump cavitation is recognized by the characteristic noise of the pump. Nowadays with advances vibration study of the pump, incoming cavitation can also be detected using harmonics. This proactive indication is opening the door to an automated diagnostic. As we discussed earlier in this blog, the traditional approach to the problem of cavitation is based on plant sizing with a static safety margin to net positive suction head required(NPSHr)in the operative range. Nowadays with the help of modern equipment, it is possible to monitor cavitation by measuring pressure, temperature and fluid speed at the nearest machine suction point. The arrival of affordable logic controllers on the market has replaced the excessive cost monitoring equipment, and today even small centrifugal pumps may benefit from such control and monitoring systems.
- The development of testing methods has consistently contributed to pump technology. The vibration measurement devices, Proximity probes, and body velocity transducers enable testing engineers to execute detailed investigations on machinery vibratory behavior and find correlations between mechanical parameters, flow readings, performance, spectrum harmonic components, orbit shapes, and specific phenomena such as cavitation or internal circulation of liquid.
- With the help of performance-based models, modern pump control systems implement the idea of complete pump automation using a programmable logic controller (PLC) systems and the minimum number of installed field sensors. This system provides machine-control, protection, and auto-diagnostic capabilities. The user sets the process parameter to be controlled, and the system provides continuous monitoring of all relevant process parameters. It automatically shifts the control over parameters that require protection intervention. When compared with traditional throttling methods, VFDs allow large reductions in the amount of power used as well as higher energy efficiencies.
- Precise pump control and better protectionVariable frequency drive VFD control system can offer protection against upset conditions like a dry run, minimum flow and cavitation with feedback from external instruments. It can calculate pump health information and protect against upset conditions by determining the pump operating condition. Smart VFDs can quickly recognize and respond to upset conditions to prevent catastrophic failure.
- Multiple pump operationTraditional multi-pump systems can be difficult to control and are often manually controlled, leading to uneven loading and wear. For example, when multiple pumps are present in a system, they can be run at the same speed but they may not deliver at the same flow and pressure. This often causes pumps to work against each other, causing inefficient or unreliable operation with high power consumption. With the help of performance-based monitoring and VFD, a controller can balance the flow output of each pump regardless of pump performance or system symmetries. This allows multiple pumps to work together optimally and decreases excess energy in the system. Ultimately this balance helps prevent failures such as seal damage, mechanical shaft damage, vibration, high temperatures, and other issues that are often a result of pumps operating against each other. Multi-pump control on smart VFDs automatically sequences pumps to match the demand and balance load evenly. It also provides automatic staging and destaging to only run the pumps necessary to most efficiently meet demands while ensuring pumps systems are balanced, leading to increased operating efficiencies, better reliability, and lower costs.
As per the cost is concerned, compared with an unmonitored installation, monitored systems with slightly higher installation costs but provide reduced maintenance costs, repair costs, and production downtime. The integration of VFDs for better operational power management will provide energy savings, maintenance cost reduction, and increased uptime. Integration of these methods is key to successful operation of next-generation centrifugal pumps.