How to specify pumps for hydrocarbon processing (2/6)

The most common types of reciprocating pumps have single or multiple pistons fitted with plungers in contact with the pumped fluid and moved by a crankshaft. Plungers are solid, not hollow, and are connected to the crankshaft outside the stuffing box by means of crossheads. The crankshaft is rotated at low speed by the electric motor through a reducing gear box. Multiple cylinders, e.g., duplex (Fig. 3), triplex, are used to increase the flow. This also reduces the pressure pulsation because strokes are staggered.

Fig3 . Duplex pump

API 674 Positive displacement pumps – reciprocating is the standard covering these pumps. They are used for moderate capacities and high differential pressures. They deliver a fixed flow; therefore, a Variable Speed Drive (VSD) must be used if the flow needs to be varied.

“Metering” or “dosing” pumps are used when a small, precisely controlled volume of liquid must be delivered at moderate to very high pressure. A typical application is chemical injection, a service for which these pumps are well suited due to the wide range of materials from which they can be fabricated.  Their design is inherently leakproof. The displaced volume is adjusted by adjusting the length of the piston stroke. The electric motor shaft has a worm periphery that rotates a gear in a plane that can be varied by a micrometre screw. The position of the plane determines the piston stroke length (Fig. 4).

Fig4 . Metering pump mechanism

The adjustment can be manual-mechanical or incorporating a fluid-filled actuator. A variable speed drive (VFD) can be used if flow variability is required.


There are two types of such pumps:

  • The plunger type, wherein the pistons are in direct contact with the pumped fluid,

  • and the diaphragm type, wherein the pistons are isolated from the pumped fluid. Diaphragm type pumps are used when the pumped fluid is of insufficient lubricity for the piston or when leak-tightness is required, e.g., for hazardous chemical or toxic liquids (e.g., ones containing H2S).

API 675 Positive displacement pumps – controlled volume is the standard covering these pumps . 

Among the positive displacement pump, some use a screw, others use gears. They are generally used in highly viscous liquids or for very low flow for which centrifugal pumps and regenerative impeller pumps are not suitable. Low viscosity liquids with poor lubricating properties (such as water) are not a proper application for gear or screw pumps. However, regenerative pumps and canned motor pumps are often capable of handling such fluids. 
API 676 Positive displacement pumps – rotary is the standard covering these pumps.


Centrifugal pumps, highlighting their main parts 

The main parts of the pump are shown in red color below.

 
Fig5 . Cross section of a back pull-out pump

Not shown, to the right in Fig. 5, is the coupling (a pair of flanges) and the driver.

Installed spare or not?


Why spend the CAPEX for installed spare pumps?  Why encounter the difficulties of operating parallel pumps?  To better understand this one should only ask the two pressing questions:

  • Is an installed spare pump required for process safety?

  • Would there be a significant loss of production without an installed spare pump?


We rarely ask these questions in Pre-FED, FEED (Front End Engineering and Design) or HAZOP.  We are missing an optimization by not addressing this point.  Sometimes the answer is self-evident.  Other times it takes research and discussion with all the stakeholders.
There are several references of chemical Plants where NO spare pumps were installed UNLESS process safety or profitability were impacted (spare pumps were supplied to be kept in the warehouse, resulting in significant savings compared to installed spare).  On average, in these process units only 10% of the pumping systems were provided with installed spares.  This “no installed spare philosophy” was mostly applied to seal-less pumps (centrifugal impeller/canned motor pumps or magnetic drive positive displacement pumps), which are easy to change-over. The philosophy would not have been applied to sealed pumps.


The CAPEX total-installed cost savings on pumping systems exceeded 75%.  Today these chemical processing units have the highest reliability and availability in the speciality chemical sector at the Operating Company (one of the world’s largest), the fewest forced outages with no fugitive emissions from pump seals.


Centrifugal pump type selection

In case there is no client requirements, leave the choice of the pump type selection to the vendor. Ask for competitive pricing and yet insist on reliability. Find the true optimum and never compromise safety.

 
 

The smallest, claimed least expensive and most efficient is the vertical inline overhung pump, OH3, shown in Fig. 6, running at 3000/3600 rpm. However, certain slightly more expensive canned motor pumps (which are now, in 2020, produced by at least one highly experienced manufacturer), may actually be a better choice. In many instances, their life cycle cost is least of all possible offers. Because the efficiencies quoted by manufacturers of conventional pumps rarely take into account the power demanded by mechanical seals, their total efficiency is often lower than that of canned motor pumps.

An in-line pump is a pump designed to be installed in-line, like a valve. It saves footprint, simplifies piping and   sometimes requires no foundation at all. The motor to pump connections are sometimes rigid or close coupled, which decreases reliability compared to horizontal overhung pumps. The radial bearings are usually located in the motor which results in higher moment and wear for the bearings and mechanical seals.

Hence, they require more frequent servicing. Close-coupled and rigid-coupled in-line pumps are therefore used for light duty, non-critical process or water service, low head and low flow. Their power is limited to 200 HP at 3,600 rpm due to vibration and to 200°C pumped fluid temperature due to shaft sealing, bearing cooling and motor cooling. A notable exception is for oil mist-lubricated bearings.  Also, there are vertical pumps with longer (7-inch) distances between motor and pump shaft ends. If properly installed and with vibratory characteristics designed-in, these superior verticals have been used with drivers up to 1,000 kW and reliabilities identical to those of horizontal pumps.


Advantages and disadvantages of vertical and horizontal pumps are shown below:

Fig6. In line pumps

The next higher cost pump is the horizontal overhung OH2 shown in Fig. 7. It is also the most commonly used type or style of pump.

A vertically (radially) split casing BB5 type is required beyond the above P, T limits. It is more expensive. They are easy to maintain as their cartridge design allow a quick replacement. BB5 type 15-20% more expensive than BB3.


High speed pumps, described in API 610 as OH6, manufactured by very few specialized vendors.

 

Vertically suspended or “VS” pumps are top mounted and have their liquid ends submerged in the pumped liquid. These are the VS1 to VS7 according to API 610 which are primarily used for service to improve NPSH available (see NPSH) or for service whose suction pressure is close to liquid’s vapor pressure. These pumps offer the advantage of being able to stack multiple impeller so they can reach very high heads. VS pumps reduce installation cost are they are mounted on their suction vessels.

 

Figure 10 below shows the most likely selection envelopes for the pump type; however, there are no strict limitations.

Sometimes this selection is not possible due to non-availability of an OH of the required size (discharge and suction diameter, impeller diameter) or appropriate speed. Limitation of OH is vibration linked to tip speed, hence impeller diameter * rpm.
In low pressure, low temperature and low power applications, BB4 type could be selected.

 

BB3 is the next cost effective choice. It cost 20-25% more than BB4. It is horizontally (axially) split and therefore easy to maintain as the top cover of the pump can be removed by lifting. BB3 can only be used up to 100 bars and 200°C.


Fig 7. Horizontal overhung (OH2) pump
Fig 8. BB4 type pump
Fig 9. BB2 type pump
Fig 10. Typical pump selection envelopes

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