Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry.

Know about Sealless CMCP (Canned motor centrifugal pump) and MDCP (Magnetic drive centrifugal Pump )

Sealless pumps ( canned type or magnetic coupling design) conforming to API 685 are very useful for chemical and oil and gas industry for pumping very hazardous and corrosive chemical liquid where leak emission to atmosphere is not acceptable .Though the initial cost of procurement is higher than the equivalent conventional/standard sealed pumps. But from long term perspective view point, cost of ownership of the sealless pumps equipped with safety monitoring instrumentation comes out out be much lower than standard sealed pumps. If safety monitoring features are not installed, it becomes a concern of reliability because nothing can be seen /observed from outside.
OEM knows the technology to design the pump to meet the intended service requirement of end user. As with all mechanical equipment, the results depend on a thorough understanding of the capabilities and requirements of the technology. A firm knowledge of end user's process requirement, along with an open relationship with a manufacturer (OEM) who thoroughly understands sealless pump technology, can yield better design results to end users. The end user must share with OEM the complete specifications such as nature of liquid, vapor pressure, volatility, density, viscosity, solid contamination in pumping liquid, besides suction and discharge conditions requirement. Inadequate specification shall lead to poor design construction resulting in reliability issue - a big concern. The one very big CONCERN is DRY RUNNING of bearings in sealless pumps the impact of which may be catastrophic in nature -a safety issue. Another big concern is air entrainment in pumped liquid which can also cause similar damaging effect as product lubricated bearing running dry. Therefore, the pump casing and containment
shell must be fully primed and properly vented prior to pump start-up. During steady state of operation, It is also very important to carryout periodical inspection at frequency well defined in service manual. This frequency is normally defined based on corrosive and erosive nature of pumped liquid and can be updated based on previous history /experience of inspection.
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Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -9

Know about range of efficiencies that can be obtained with centrifugal pumps

The efficiency of the pump is function of many factors e.g. specific speed, its relative size, the service for which It is intended, its material of construction, physical characteristics such as viscosity of the pumping liquid. Large centrifugal pumps have developed over 92 per cent efficiency. Small pumps handling viscous liquids, under severe mechanical conditions calling for relatively large clearances between the wear rings, may have efficiencies as low as 10 or even below 5 per cent.
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Knowledge sharing on proper gasket installation

 To my experience, I have analyzed the root cause of gaskets failure was mostly the lack of knowledge or competency to know about good practices of installation of gaskets in between piping flanged joints or in vessel nozzle joints. We must agree that all these gaskets are procured as per specifications furnished by licensers/piping or equipment designer. They select the proper type of gaskets based on service conditions and also on "how much equipment and piping system is safety CRITICAL." I believe all end users do these.

But again why gasket fails??. We must analyze it. Before Fixing of these right size and right rating gaskets in their locations, it needs some basic checks ( known as Last chance Inspection) at site location e.g. whether companion flanges are having any sort of misalignment beyond the acceptable limit? or the gap between flanges proper ? or they adequately (properly) torqued or they have short bolting ? All such abnormality must be addressed prior to installation of gasket.. Therefore all good organizations follow the practice of developing BEP and SMP ( Standard Maintenance Practice) for installation of proper gaskets in the field.
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Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -8

 

Let us know about how does viscosity of a liquid affect the selection of centrifugal pump?

As we know that disk friction varies as the fifth power of the diameter of the impeller but only as the speed cubed, it is desirable to use the highest speed (and the resulting smaller impeller diameter) consistent with design conditions.

Head loss due to viscosity increases as operating conditions approach the best efficiency point of the pump because of increased velocity through the impeller.

Therefore, there is an advantage in selecting a relatively oversized pump for highly viscous liquid so as to place the operating conditions well to the left of capacity at best efficiency on low viscosity service.

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Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -7

 

Know about different kind of losses in a centrifugal pump with their brief descriptions.

Losses in a CP are classified as:

1.   Hydraulic loss: These are caused by shocks, eddy currents, and friction of the fluid in its path through the impeller and casing waterways.

2.   Disk-friction Losses: These are commonly known as “Disk horse-power” which represent the power required to rotate the impeller in the liquid surrounding it in the pump casing. With other factors constant , the disk horsepower increases rapidly with the viscosity of pumping liquid.

3.   Short-circuit losses: They are losses caused by leakage from the discharge to the suction side of a pump through the clearances between the casing and impeller wear rings. This leakage reduces the effective capacity of the pump because the leakage has to be re- pumped from suction to discharge.

4.   Mechanical losses: They are losses resulting from friction in the stuffing boxes and the pump bearings

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Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -6

Know about balancing disk and its function how does it balances the axial hydraulic thrust in multistage centrifugal pumps.?

The rotating part of an axial-clearance hydraulic balancing device (Fig-1) is a balancing disk and the stationery part is known as balancing disk head.

The balancing disk is fixed to and rotates with the shaft, being separated from the balancing disk head by small amount of axial clearance. Leakage through this clearance flows in to the balancing chamber and to the pump suction or suction source.

Balancing chamber pressure exists on the back of the balancing disk, while its face is subjected to an intermediate pressure between discharge pressure at the smallest diameter of the disk and the balancing - chamber pressure at its periphery. Inner and outer disk diameters are so designed that the difference between the force acting on the face of the disk and that on its back balances the axial thrust of the impellers.

If the axial thrust of the pump increases, the excess pressure moves the rotor to reduce the clearance between the balancing disk and disk head. This reduces the flow through the clearance and lowers the balancing chamber pressure until the pump axial thrust is again balanced. Similarly, a reduction in axial thrust causes an increase in clearance between the disk and the disk head. As a result of increased flow, pressure increases in balancing-chamber until the axial hydraulic thrust is again balanced.

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                                 Fig-1.Balancing disk and balancing chamber arrangement

 

Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -5

 When are self -aligning ball bearings and self aligning roller bearings used in centrifugal pumps?

For heavy loads, high speeds, and long bearing spans with no end thrust, self-aligning ball bearings  can well be adopted for line bearings on centrifugal pumps. The double row of balls runs in fixed grooves in the inner or shaft race, and the outer race is spherically ground (Fig-1); Therefore it operates as a pivot to take slight vibration or shaft deflection. It has very little thrust load (axial load) capacity and hence is not used for combined radial and thrust load in centrifugal pumps. With large shafts, the self- aligning spherical roller design is used instead of self- aligning ball bearings.

Self-aligning spherical roller bearings (Fig-2) can carry a combined radial and axial load with considerable thrust load component.

Fig-1.Self-aligning ball bearings

Fig-2.Self-aligning spherical roller brg.

Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -4

1. Relationship between a critical speed of a pump shaft rotor          and its operating speed.

Generally, the pump shaft rotor dimensions of a single stage pump cause the first critical speed to be considerable above its operating      speed. However, centrifugal pumps may operate above their first        critical speed without danger if the speed is high enough. This is        done by OEM, in multistage pumps by designing their shafts so          that the first critical speed is from 60 to 75% of the operating              speed.

 

2. What is meant by rigid and flexible shafts?

A rigid shaft has a first critical speed higher than its operating speed.

 A flexible shaft has its operating speed higher than one of its critical speeds

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Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -3

 

To know about Volute type Centrifugal pump

A pump named from its spiral-volute form of casing, which acts as collector of fluid discharged by the impeller. Wall T, dividing the beginning of the volute and the discharge nozzle, is called the “volute tongue” or “cut water”

Knowledge sharing on “All about Centrifugal Pumps” for Engineering students/young Engineers/professionals of oil & gas industry. Series -2

 

To know about how centrifugal pumps are classified?

 

 They are classified according to

   1. types of energy conversion such as

        1.1 Volute type

        1.2 diffuser or turbine type

   2. number of stages - single stage or multistage.

   3. Impeller type such as single or double suction, open, semi open or closed impeller,

        single or double curvature vanes

   4. Axis of rotation: Horizontal, vertical or inclined

   5. Casing type: Split or solid  location of suction and discharge nozzles

 There are other classifications such as classes of service, features of

 construction, and rotation

Correlation between hardness and tensile strength of Engineering steels materials

 Ductility and brittleness: Ductility is ability to undergo plastic deformation under given increase in load to the extent of fracture. However Brittle materials experiences very little or almost zero plastic deformation when subjected to tensile load beyond elastic limit and fractured at fracture strength which is close to its elastic limit or yield point.

Generally, a material can be considered as brittle, if it is strained up to less than 5% at the point of its fracture. Schematic representation of ductile and brittle materials sustained to deformed at fracture point is shown in the sketch below. Though the brittle material has higher yield stress and tensile strength as compared to ductile, it is not tougher than ductile

As a rule of thumb for most of the steels, tensile strength  is related to following formula :

 1.Tensile strength ( psi) = 500* BHN

 2.Tensile strength ( MPa) = 3.45* BHN

Knowledge sharing on “All about Centrifugal Pumps” for young Engineers /professionals

 

Series-1 . Essential structural elements of any centrifugal pump :

They are rotating element comprising basically a shaft and impeller, and the stationary elements comprising of casing, stuffing boxes and bearings. All other parts are refinements of construction supplementing the function of the main elements.

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Figure -1 given below indicates the details of all the parts.

Fig-1 : Axial cross section of single stage, single suction impeller, split casing  with side suction and side discharge  nozzles of a centrifugal pump

Troubleshooting of process machinery is not only a science but an art also.

 

Rotating machinery commonly used in various processing plants, such as centrifugal pumps and compressors, reciprocating pumps and compressors, fans/blowers, steam turbines, and electric motors.

Simple troubleshooting tables as advised by OEM or decision trees are rarely effective in solving complex, real-world machine problems. For this reason, the Petchem Skills Consulting services( https://petchemskills.com) offers a novel way to attack those machinery issues that can adversely affect the safety, reliability and efficiency of plant processes. The methodology adopted and being taught to the Reliability Engineering and asset management professionals is not a rigid approach but rather a flexible and dynamic process aimed at exploring process plant machines holistically in order to understand and narrow down the true nature of the problem.

We train and guide maintenance and machinery professionals and managers to better understand how to troubleshoot process machinery in -situ i.e., in the field when machinery is in the running condition. This we call it “Field troubleshooting” which means and involve operation and maintenance engineers/ supervisors to ask themselves “Is My Machine running  OK”? using the “who, what, when, where, why” troubleshooting methodology.

To be successful, the field trouble-shooter must be persistent, open-minded and disciplined. Once field data is collected, an unbiased, logical approach to the finding is required to hone in on the most probable source of an observed symptom (or symptoms). Without a comprehensive and logical analysis of the findings, the investigator is only guessing, which wastes valuable time and resources.

We know that Process machines are critical to the profitability of processes. Safe, efficient and reliable machines are required to maintain dependable manufacturing processes that can create saleable, on-spec product on time, and at the desired production rate. As owner of the process machinery, one must wish to keep  equipment in serviceable condition all the time.

One of the most challenging aspects of a machinery professional or operator’s job is deciding whether an operating machine should be shut down due to a perceived problem or be allowed to keep operating and at what level of operation. If he or she wrongly recommends a repair be conducted, the remaining useful machine life is wasted, but if he or she is right, they can save the organization from severe consequences, such as product releases, fires, costly secondary machine damage, etc. This economic balancing act is at the heart of all machinery assessments.

The primary purpose of the knowledge and skills taught by us to guide help operators and machinery professionals troubleshoot machines that are in a process service and operating at design process conditions. Hence the definition of Field troubleshooting is a process of determining the cause of an apparent machine problem, i.e., symptom, while it is still operating at actual process conditions. Troubleshooting efforts tend to focus on a specific machine or subsystem, using a proven body of historical knowledge. The body of knowledge may be in the form of troubleshooting tables and matrices or manufacturer’s information. Keep in mind that process machinery can only truly be tested and evaluated in service and under full load, i.e., in- situ Very few testing facilities are available that can test a pump or compressor at full process loads and with actual process fluids. Field troubleshooting evaluates the mechanical integrity of a machine in process service in order to determine if symptoms are the result of an actual machine fault or a process-related problem.

Here are examples of troubleshooting opportunities:

Example 1: Pump flow has fallen well below its rated level.

Example2: Compressor thrust bearing is running 10 °C hotter than it was last month.

Approach to be adopted by field trouble-shooter :

1. The field trouble-shooter must first ask: Do I fully understand the machine or subsystem that needs to be analysed? If the complexity is beyond the trouble-shooter’s abilities, he or she should get help. At this point, management may decide to conduct an RCA analysis.

2. If the field trouble-shooter decides to tackle the problem at hand, he or she should then ask: “Are the observed symptoms caused by a failing machine, a correctable fault, or by undesirable process conditions?” If it is a process related problem, changes can be made before permanent machine damage occurs. If a fault is deemed to be correctable, then adjustments or minor repairs can be made in order to quickly restore the machine to serviceable conditions. This what is expected from the field trouble shooter

If the machine fails, either a failure analysis or root cause failure analysis must be performed, depending on the extent and cost of the failure.

Challenges faced by OEM in ensuring Reliability and optimizing the manufacturing cost of Centrifugal pumps for process/oil & gas industry.

Big challenge is determination of kind of materials of Construction of Centrifugal  Pump  to ensure the long-term Reliability to establish reputation of business with their potential clients in the market.

Wish to highlight here following points, to the best of my understanding, which process industry professionals may like to know. Centrifugal pumps are manufactured of almost all known common metals or metal alloys as well as of carbon, porcelain, glass, stoneware, hard rubber and even synthetics. Service Conditions and the nature of fluid pumped determine the most satisfactory materials. Some of the factors that enter into the selection of materials are given below:

1.    Discharge pressure

2.    Pumping liquid temperature

3.    Abrasiveness of suspended solids in the liquid

4.    Corrosion resistance

5.    Electrochemical action

6.     Head per stage

7.    Suitability of material to the structural features of the particular pump involved

8.    Load factor and expected duration of pumping installation

 

Monitoring Submersible motor pump  health for water application

Check points (monitoring points) by pump operator ( Look Listen Feel and Record Approach)

by pump operator:

1. Observe and Record the discharge pressure gauge reading (must have been installed in discharge

piping system).

If the reading is significantly oscillating (hunting to and fro) more than 15-20% of normal

operating point, it gives following signals which can be addressed through skilled service

technicians:

1.1. Strainer might have been choked due to mud /debris/scales accumulation around the

strainer. Needs cleaning of strainer.

2. Abnormal noise coming out from the piping system which is a signal of Cavitation phenomenon

in the system. This may be due to starvation in the pump. Remedy suggested is same as in point

no 1.1 above. OR level of water in underground water tank is lower than strainer location.

Pumps need not to be started unless level in water tank is adequately maintained.

3 Pump is running but with no flow or significantly reduced flow and with shut off pressure ( which

is more than operating pressure):

This indicates that NRV installed in discharge piping system is jammed ( not opening ) .Needs

inspection of NRV.

4.Pump is getting tripped after start of few seconds . This gives signal that

4.1 Either NRV is heavily passing ( WORN OUT FLAPPER ) and discharge valve is in full open

condition. Leading to over load of pump motor . OR

4.2 Pump motor bearing is damaged .

Measurement of motor current will confirm the over loading . This Needs inspection of NRV.

5. Pump is not getting started at all. Following needs to be checked:

5.1 IR value ( Insulation resistance) of wiring of motor. If IR value is less than 0.5 MegaOhm

needs inspection of wire /winding to fix the issue by skilled SERVICE electrician.

5.2 If IR value is within normal limit, still pump is not getting started then it needs inspection of

pump internals for bearing seizure with motor shaft either due to increased clearances or due to

lack of cooling by water as lubricant/coolant. Submersible pump sleeve bearing is water lubricated.