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Part 4 of 4

It is good practice to initially fill the pump and as much of the inlet piping system as possible with the liquid to be pumped. This will assist in priming and reduce the risk of pump damage during an otherwise dry start. A rotary pump will prime more quickly if internal pumping elements are at least wetted. Priming is nothing more than pumping air from the inlet system to the discharge system. The ability of a rotary positive displacement pump to act as an air compressor is very much related to having some liquid present internally.

Pump shaft seals, especially mechanical seals, should never be operated dry. Immediate, or at best premature, seal failure is the inevitable result. Again, filling the pump with the liquid to be pumped and hand rotating the pump a few times helps ensure that liquid is present at the shaft sealing mechanism to carry away frictional heat during startup. If the particular pump has a seal chamber access plug, remove the plug, fill the chamber with liquid and reinstall the plug.

Have phone numbers of key vendor service departments, fire brigade and medical emergency services on hand in the event they are needed. When handling petroleum and other flammable liquids, there are both pollution and fire hazards present.

Ensure that there is an adequate supply of liquid in the pump inlet system (no half empty supply oil tanks or the like). It is also prudent to confirm where pump discharge flow will be going to be sure the discharge system is ready.

Loud or erratic noise at startup is an indication of cavitation (inadequate pump inlet pressure) or air being drawn into the pump inlet system. It is frequently accompanied by increases in or excessive vibration. If mild, troubleshoot the cause. If severe, shut down the pump and find the source of the problem.

Use the ROTARY PUMP STARTUP CHECK LIST or a similar control to help ensure that all contingencies have been addressed.

CONCLUSION

Our discussion cannot be considered all-inclusive since each pumping system has unique features and requirements, some of which may interact with each other or with other aspects of the overall plant operation. In addition, no allowance has been made here for regulatory requirements, specialised industry or company guidelines or the like. Where values are recommended, they are intended for use in the absence of vendor or specifically engineered information. Always use the more stringent of either the recommendations herein or the vendor or engineer’s guidelines.

ROTARY PUMP

Project:__________Location:__________Unit No.:__________Tag No.:_________

1. PIPING 7. SPARES AVAILABLE

❏ Clean ❏ Pump

❏ Bolts Tight ❏ Driver

❏ Strain Removed ❏ Gear

❏ Gaskets in Place ❏ Other__________

❏ Pressure Tested

❏ Flushed 8. RESOURCES AVAILABLE

❏ Other__________ ❏ Electric

❏ Steam

2. VALVES ❏ Cooling Water

❏ Not Backwards ❏ Hot Oil

❏ Clean ❏ Auxiliaries

❏ Bolts Tight ❏ Gages in Place

❏ Gaskets in Place ❏ Other__________

❏ Correct Position (Open/Close)

❏ Relief Valve Set Pressure 9. LAST MINUTE

❏ Other__________ ❏ Pump Filled with Liquid

❏ Shaft Seals Wetted

❏ Key Contact Phone Nos.

3. FOUNDATION ❏ Inlet Liquid Supply

❏ Level ❏ Discharge System Ready

❏ Solid (No Voids) ❏ Air Bleed Valve Open

❏ Bolts Tight ❏ Pump Hand Rotated

❏ Other__________ ❏ Other__________

10. COMPANY/INDUSTRY SPECIFIC

4. ALIGNMENT ❏ _______________

❏ Angular (Cold & Hot)_____ _____ ❏ _______________

❏ Parallel (Cold & Hot)_____ _____ ❏ _______________

❏ Other__________ ❏ _______________

❏ _______________

5. ROTATION

❏ Verified (CW or CCW)

❏ Other__________

6. LUBRICATION

❏ Pump

❏ Driver

❏ Gear

❏ Other__________

7. SPARES AVAILABLE

❏ Pump

❏ Driver

❏ Gear

❏ Other_______

8. RESOURCES AVAILABLE

❏ Electric

❏Steam

❏ Compressed air

❏Cooling water

❏ Hot oil

❏ Auxiliaries

❏ Gages in place

❏ Other_________

9. LAST MINUTE

❏ Pump filled with liquid

❏ Shaft seals wetted

❏ Key contact phone numbers

❏ Inlet liquid supply

❏ Discharge system ready

❏ Air bleed valve open

❏ Pump hand rotated

❏ Other____________

10. COMPANY SPECIFIC

❏ _________________

❏ _________________

❏ _________________

❏ _________________

❏ _________________

For more information, contact Jim Brennan at

LUBRICATION

Most rotating machinery has some form of lubrication for its bearing systems, figure 2. It may be as simple as a permanently grease-packed, sealed ball bearing or as complicated as a separate lubricating oil pump system complete with cooler, filter, instrumentation, etc. Be sure to verify that any lubrication required has been addressed. Equipment having been in storage may require draining and addition of fresh lubricant or even flushing out of preservatives before fresh lubricant is added. Any gearing present (pump timing gears, reduction drive gears, etc.) should be reviewed for the presence of the correct type and quantity of lubricant. Constant level oilers should be filled to their mark with clean, fresh lubricant of the correct type. Some flexible couplings are grease lubricated and should also be checked. Most electric motors will have grease-lubricated, antifriction bearings that should be checked as well.

Almost all rotary pumps should be able to be turned over by hand. They should generally turn over smoothly, with no catches or uneven rubbing. Very large pumps may need a helper bar but should not be at all difficult to turn. If not, consult pump vendor. Partial disassembly may be advisable to determine the cause of difficulty encountered (foreign material, pipe strain, rust, etc.) before starting.

STARTUP SPARES

With care and planning, startups will generally go smoothly, without significant problems. However, it is prudent to have key spare parts on hand in the event they are needed quickly for correction after some unanticipated problem, minor damage, or need to disassemble a piece of equipment for inspection. For rotary pumps this would normally be a set of shaft seals, gaskets, “o” rings and bearings, frequently available as a minor repair kit. For other rotating equipment, spare bearings, grease and oil seals, gaskets, etc., should be on hand so as to avoid delay in the pump startup. More extensive spares will depend on availability from the vendor, criticality of pump operation, plant practice and, perhaps, other issues specific to the installation. If the startup goes well and the spares are not consumed, they are very appropriate to be held on hand for future routine inspections and service.

RESOURCES

Be sure electric power, steam, cooling water, hot oil, instrumentation power or air or any other auxiliary resources are available and ready before start. Be sure adequate pressure and temperature gages are in place so observations can be made during startup. Without them, you are working blind. Speed indication (tachometers) may also be needed if the drive is not a fixed speed one such as an AC electric motor. If the pump will be handling hot liquid, preheat the pump as necessary so it is not exposed to thermal shock when otherwise hot liquid reaches an ambient temperature pump. Rotary pumps may be somewhat more sensitive to thermal shock due to their close internal running clearances.

 FOUNDATION, ALIGNMENT AND ROTATIONIf horizontal pumps are used, be sure the foundation is level, hold down bolts are tight and grouting, if used, has completely filled the baseplate (no hollows or voids) and has cured. If the pump will be handling liquid above about 150 °F (65 °C) or a steam turbine is used as the driver, an estimate of the centerline growth in height of the hot machine must be made. Shaft to shaft alignment (cold) should incorporate a deliberate, compensating off set, so that alignment is more nearly correct when equipment is up to operating temperature. Coefficients of thermal expansion for common pump case materials are:

Thermal Growth Coefficients (x10^-6)

Material inches/inch/°F mm/mm/°CCast iron 6.0 11.0

Ductile iron 6.6 12.0

Cast steel 6.5 12.1

316 Stainless steel 9.4 17.0

The coefficient is applied to the centerline height of the shafts and the difference in temperature between that at which the unit was aligned and temperature of expected operation. The cold machine should be shimmed high by the above calculated amount.

The purpose on any shaft aligning procedure is to align the centers of the machine shafts with each other, NOT to align the flexible coupling hubs. At temperature, alignment should be within 0.003 inches (0.076 mm) Total Indicator Reading (TIR), both angular and parallel. Consult a good aligning procedure to achieve or verify this degree of precision. The fact that the coupling may be rated to a much greater misalignment capability has nothing to do with the shaft-to-shaft alignment of the equipment. Survival and longevity of the machinery, NOT the coupling, are the objectives. If hot pumps and/or drivers are used, after they are at nominal operating temperature long enough for thermal growth to have stabilized (probably one hour or more), shut down the equipment and verify that alignment is within proscribed limits.

Never rely upon the alignment that was produced where the pump and drive train were assembled. Transportation, lifting and handling as well as foundation irregularities will impact alignment, always in an undesirable direction. Final alignment should be achieved as nearly the last step before actual starting of the pump. If equipment is to be dowelled in place, do so to the pump ONLY after several hours, if not days, of good operation and hot alignment checks.

The use of resilient mounts is sometimes desired to reduce vibration being transmitted into the underlying foundation. If used, such mounts must not be used beneath the pump or driver but between the pump/driver baseplate or bracket and the foundation. The pump and driver must be rigidly aligned, not resiliently aligned, as the resilient mounts will not maintain adequate alignment under torsional reactions from the transmitted torque.

Direction of rotation is critical for most equipment. It is usually indicated by arrow nameplates. Remember that some gearing will reverse rotation from input shaft to output shaft. Most engines and turbines must be purchased for a specific direction of rotation. This is also true of most pumps. Standard AC electric motors are frequently bidirectional; their direction of rotation will depend upon how the power cables are connected. It is normally not possible to predict their direction of rotation before hand. It is recommended that the flexible coupling at the motor shaft be disconnected and the motor momentarily energized (jogged on, then immediately off) to see if its rotation is correct for the rest of the driven equipment. If not, two of the electric power cables will need to have their connections reversed. Verify correct rotation after reversing, if necessary, before re-engaging the flexible coupling.

Check back in a few days for additional content on rotary pump startups.

For more information, contact Jim Brennan at

• Avoiding Problems and Maximizing Operation During Rotary Pump Startups 

  Many pump startups are the culmination of months, if not years, of work designing the process, machine or system; specifying components, instrumentation and protective devices; and reviewing and qualifying suppliers, etc. It is also the most vulnerable time for any pump. This article describes cautions, reviews and inspections that should be conducted before startup to help ensure that all those many gremlins of pumping systems are found out and addressed in time.

  To begin with, thoroughly read the technical manuals and instructions from the pump, driver and all auxiliary equipment suppliers to uncover requirements that may be specific to their equipment design. This is the easiest method to protect the system but is overlooked more often than not.

  PIPES AND VALVES

  Piping and valving installation, figure 1, should probably be considered first. Be sure all required valves have been installed. Verify that none are installed backwards. An absent or reverse-mounted check valve, foot valve or relief valve can cause some very serious damage. Piping should have been inspected during fabrication to ensure that weld bead, weld rod, scale, etc. have been completely removed. Such hard particles can cause catastrophic pump failure if they lodge in the wrong pump clearance. Temporary, if not permanent, pump inlet strainers should be considered if not already present. They should start in a clean condition so that accumulation of dirt can be monitored.

  The piping system should be pressure tested. Avoid imposing on any system component pressures in excess of their design limits. Many pumps can withstand discharge pressure only on their discharge side. Inlet piping systems are frequently suitable only for low pressure. The pressure test medium should be compatible with the components/system be tested. Don’t use water if the system is not a water system. A low-pressure (15 PSIG, 1 Bar g) compressed air test may be adequate to find missing flange gaskets or other obvious leak sources.

  Check and tighten all flange bolts to specified torque. Pump inlet and discharge piping should have been made up from the pump for a distance of perhaps 20 feet (6 meters) to minimize pipe strain on the pump. Piping should be independently supported. Close internal clearance positive displacement rotary pumps do not make very good pipe anchors. When pipe flanges are unbolted from the pump, flange bolts should be able to be installed/removed without forcing piping into position. Additionally, there should be a flange-to-pump gap not exceeding the greater of twice the flange gasket thickness or 1/16 inch (3 mm).

  Positive displacement pumps will normally have a system pressure-relief valve installed from the discharge piping to either the source of the pumped liquid, such as a supply tank, or to the pump inlet piping (a less desirable point due to the potential for temperature build up during relief valve operation). This valve will normally be set slightly higher than the maximum anticipated normal system operating pressure. If possible, verify that it has been properly set. If this cannot be verified, consider adjusting the relief valve to a very low pressure and changing it upward after pump startup. Consult relief valve vendor’s technical data to be sure valve adjustment is done in the correct (to lower pressure) direction.

  Ideally, the entire piping and valve system will be thoroughly flushed to remove all dirt and fabrication debris. This is normally done using a flush pump, not the normal system pump. Strainers and or filters are installed at appropriate locations and their dirt accumulation is monitored until they show no accumulation for a period of 24 hours. Flushing usually uses light, fairly hot (150 °F, 65 °C) oil delivered at flow rates higher than system design. The higher flow rates cause higher liquid velocities within the piping system and are more likely to dislodge debris.

  Some systems will also use vibration equipment to impose mechanical “shaking” on the piping, again to improve dislodging dirt. Very extensive piping systems have been known to still show debris accumulation after 30 days of flushing. Pipeline systems, due to their long distances and relatively huge holding volumes, will frequently use “pigs,” bullet-shaped devices, sometimes equipped with wire bristles, which are propelled ahead of a flush or initial product batch of liquid to scrub debris and dirt from the pipeline inside diameter.

  Before final startup, be sure valves are open or closed as required. Pump inlet and discharge valves are normally left full open. Manual pump bypass valves are also normally left open on startup. An air bleed valve in the discharge piping at a high point near the pump will significantly improve the pump’s ability to self prime. The valve is left open during startup until liquid flows. It is then shut. Be sure to know where this flow will be directed to avoid inadvertent discharge to atmosphere or spillage. Steam turbine steam valving is very important. Turbine startup procedures should be thoroughly reviewed as there are personal injury issues associated with this equipment if started or operated improperly.

  Check back in a few days for additional content on rotary pump startups.

  For more information, contact Jim Brennan at

   

   

  jimb@pumpxpert.com, and visit www.colfaxcorp.com

  .

Precision timing gears can operate in the 98% range. The mechanical seal drag has a component of loss for the body rotating within the fluid as well as a component due to the shearing of the liquid film between the rotating face and stationary seat. All these losses are normally very small and might contribute only a few percentage points of inefficiency. This is true unless pumps are operated at very low hydraulic power levels.

As is true with most rotating equipment, larger machines are more efficient. In the case of screw pumps, the reason is that the theoretical flow rate is a function of the cube of the screw size while slip flow, everything else being constant, is a function of the square of the screw size. Figure 5 illustrates this effect for an 8 wrap, 1000 PSID, 1200 RPM screw pump handling 100 centistoke (500 SSU) crude oil.

The determination of pump efficiency is a straightforward calculation as follows:

E0 = Power out / Power in = (QD X PD X 100 / k) / W

E0 = EV X EM X 100

EV = QD / QT X 100

UNITS

E0 = overall pump efficiency % % %

EV = pump volumetric efficiency % % %

EM = pump mechanical efficiency % % %

QD = pump delivered flow rate GPM B/D M3/H

PD = pump differential pressure PSI PSI BAR

k = conversion constant 1714 58764 36.03

W = pump input power HP HP KW

QT = pump theoretical flow rate GPM B/D M3/H

Since pumps are frequently sized to operate over a range of pressure and viscosity, for cost calculations, use the power required at the pressure and viscosity that will be representative of normal operation. Screw pumps will require their maximum input power at maximum viscosity. The minimum flow will be delivered at the minimum viscosity. Do not use the minimum delivered flow and the maximum required power to calculate overall pump efficiency. This method understates the efficiency as simultaneous operation at these conditions is not possible.

Figure 6 shows a three screw crude oil emulsion shipping pump on a California offshore

platform. There are three pumps on each of two platforms. Each pump delivers 800 GPM (27,500 B/D) at design discharge pressures to 1190 PSIG. The 800 HP, 1200 RPM electric motor drivers were sized to handle a maximum pumping viscosity of 350 centistokes (1610 SSU). The overall pump operating efficiency at this point is 82%, considerably better than available from centrifugal equipment.

(87,500 B/D) of Orimulsion, an emulsion of 30% water and 70% bitumen, that is exported as a power plant fuel. Design differential pressure is 531 PSID and the pumping viscosity range is 215 to 970 centistokes (1000 to 4500 SSU). The pumps are driven by 1250 HP, 1200 RPM electric motors and operate in the 78 to 82% efficiency range.

Figure 8 shows the range of multiple screw pump sizes available for transport of crude oil.

In order to maintain high efficiencies over longer time periods when pumping crudes with sand, carbonates, sediment, etc., screw pump manufacturers use a number of techniques to enhance the life and prolong the running clearances within these pumps. Hardened or hard coated screws, hard chrome plated liner bores, hard/soft and hard/hard combinations of running surfaces and erosion resistant inlays and overlays all can be used to contribute to longer useful pumping life between overhauls.

SCREW PUMP CENTRIFUGAL PUMPFlow 583 GPM (20,000 B/D- 132 M3/H)

Pressure 1000 PSI (70 BAR)

Crude Viscosity 1000 SSU (200 cst)

Efficiency 82% 45%Power Req’d. 415 HP (309 KW) 756 HP (563 KW)

The power difference is 563 – 309 or 254 KW. With 8760 hours in a year, the annual

energy difference is 254 X 8760 or 2,225,040 KW-Hr. At an energy cost of $0.104 / KW-Hr (2008 U.S. average), the annual direct energy cost difference is $240,000. Add the cost of carrying money, currently about 10%, and the difference is over $268,000. per year for one pump, a significant amount of money.

jimb@pumpxpert.com, and visit www.colfaxcorp.com

  How Screw Pumps Can Achieve 80%-Plus Efficiency in Crude Oil Pipelines, Part 2 of 3Multiple screw pumps for crude oil pipeline service are produced in two basic configurations. The twin screw pump is illustrated in figure 1. It is a double suction design. Each shaft is independently supported with bearings external to the pumped liquid. The mesh of the screw set is synchronized through the outboard timing gears; the screws do not touch each other.

Figure 2 shows the construction of a three screw pipeline pump of the single suction design. It incorporates a replaceable liner and single, external support bearing. The smaller outside screws, called idler rotors, are driven by the center screw, called a power rotor. Thus there is metal to metal contact and these designs cannot be used with high gas content or for 100% water. Each type has its merits depending on pressure, flow and the condition of the crude oil.

Screw pumps are positive displacement machines. Every revolution of the pump shaft causes a specific volume of space to be opened to the system inlet pressure environment and then closed off from the inlet. The volume moves in an axial direction and is expelled from the pump by the next succeeding volume. Flow is very smooth and almost completely free of any measurable pressure or flow pulsation. At constant speed, screw pumps have a theoretical displacement dependent upon the size and geometry of the screws and the screw pitch or lead.

The pumps, obviously, have internal running clearances and will not be able to deliver 100% of their theoretical flow when pumping against a differential pressure. Slip flow occurs through the running clearances. The slip flow is a function of differential pressure and fluid viscosity. Increasing differential pressure and decreasing viscosity cause slip to increase. Slip flow is the volumetric inefficiency.

A pump having a theoretical flow of 432 GPM operating at 1000 PSID and 20 centistokes (100 SSU) might have a slip flow of 59 GPM. Thus the pump would deliver 432-59 or 373 GPM. The volumetric efficiency of the pump would be 373/432 or 86%. At higher viscosities, common on crude oils with an API gravity less than 200, the volumetric efficiency of a multiple screw pump can reach well into the 90 to 95% range.In order to limit the slip flow characteristics of multiple screw pumps, higher pressure designs use more “wraps” of screw thread than lower pressure designs. Each wrap acts as a barrier to slip flow, effectively causing the pump pressure rise to occur in stages, figure 3. The staging effect lowers the loading on rotating pump components as well as providing greater resistance to slip flow.

At low viscosities, slip flow is the major contributor to the inefficiency of a multiple screw pump. At increasing viscosity, the slip flow is reduced, sometimes to a negligible level. However, as viscosity increases, more power is required to rotate the pumping screws within their close clearance stationary boundaries. Viscosity is defined as a liquid’s resistance to shear. The pumping screws shear the liquid that is within the running clearances and this is the major contributor to inefficiency when operating at high viscosity.

Proper selection of pump size and speed can keep these viscous shear losses within reason and usually not require and pump speed reduction devices except under the most severe conditions. Figure 4 shows the performance curve of a typical crude oil pipeline screw pump at constant speed. The effects of viscosity and differential pressure are clearly evident.

Note that, like all machines, the pump efficiency is zero at zero differential pressure. This is the point where the machine would pump its theoretical flow but since there is no pressure rise, there is no power output yet it requires about 15 HP minimum to keep the pump rotating. Also note that there is no “best efficiency” point as with a centrifugal pump. Multiple screw pumps have a rapidly rising efficiency curve which then holds fairly high throughout its design pressure range.

Check back in a few days for additional content on how screw pumps are an efficient option for crude oil pipelines.

jimb@pumpxpert.com, and visit www.colfaxcorp.com

Boosting and pipelining heavy crude oil is a classic example of the operating cost benefits of using screw pumps over conventional centrifugal pumps. Considering oil prices over the past year, production and transportation operating costs are more important than ever and likely to remain that way well into the future. Screw pumps are uniquely suited to this service, offering pump efficiencies in the 80% range while requiring little, if any, additional crude oil heating or dilution.

Power losses within multiple screw type pumps include volumetric, viscous and mechanical. The staging effects of screw pumps help minimize volumetric losses. Viscous shear losses are controlled by careful selection of operating speeds, typically in the 900 to 1800 RPM range. Mechanical losses from seals and bearings are usually a very small percentage of the total losses. Larger machines, including screw pumps, improve efficiency.

Multiple screw pumps have been used on heavy oil pipeline services since the 1950′s. Their capabilities and range of capacity has steadily improved and are now able to provide reasonable life on this demanding service while offering excellent operating efficiencies.

Check back in a few days for more detailed content on how screw pumps are an efficient option for crude oil pipelines.

For more information, contact Jim Brennan at

  Series Crude Oil Pipeline Stations Using Rotary Positive Displacement Pumps 

The initiating station will have storage tanks/tank farm from which this station draws oil, sometimes via a low pressure boost pump unless the high pressure pumps are in close proximity to the tanks. The most common arrangement uses three 1/2 capacity pumps (one or two running, one a hot spare). This first station pumps the oil to the inlet of the next station. The receiving station has a bypass control valve from station discharge to station inlet. The bypass control valve senses station inlet pressure and bypasses whatever flow is necessary to maintain station inlet pressure at set point, usually 50 to 75 PSIG (3.5 to 5.2 BarG).

Normally, one of the second or subsequent station pumps is started. If station inlet pressure rises above set pressure, a second pump is started and the bypass control valve recirculates whatever volume of oil is necessary to maintain station inlet pressure at set point.

An alternative approach uses a variable speed drive to control pump speed maintaining station inlet pressure at the desired point. Since any combination of pumps could be running at the same time, all of the pumps should be equipped with variable speed drivers.

Regardless of method, each pump needs its own pressure relief valve (not shown below for clarity) connected from pump discharge to pump inlet. Their set pressure needs to be about 8 to 10 percent above normal station discharge pressure.

I attended the fourth annual PumpTec conference (Holiday Inn Select on Peachtree Industrial Drive) in Norcross (Atlanta), GA, USA last week. There were 45 attendees taking the technical courses, some from as far away as the Philippines. The symposium was a two day event covering such topics as:

End suction centrifugal pumps by Lev Nelik, Pumping Machinery LLC

Double suction centrifugal pumps by Pete Noll, Peerless Pumps

Vertical turbine pumps by Rick Silcox, Peerless Pump

Internal and external gear pumps by John Petersen, Viking/Idex

Progressive cavity pumps by Jim Siebolt, Colfax Corp.

Multiple screw pumps (two screw and three screw) by Jim Brennan, Colfax Corp., retired

The symposium included “hands-on” disassembly and assembly of pumps by attendees as well as NPSH demonstration testing of small centrifugal pumps. Tuesday afternoon included Bearings and Lubrication and Alignment and Vibration Fundamentals.

PumpTec 2009 will be held September 14 & 15, 2009. For additional information, go to www.PumpingMachinery.com.