LOW NPSH & CASES OF VERTICAL BARREL (VS6) TYPE PUMP SELECTION

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LOW NPSH & CASES OF VERTICAL BARREL

(VS6) TYPE PUMP SELECTION 18 MAR 2015

By: Muhammad Imran – Mechanical Rotating Engineer – LUKOIL Overseas Services B.V Dubai

rate (Q – m3/h), Differential Head (H – m) and system Net Positive Suction

Head (NPSH – m) are the prime parameters which govern pump type selection

(Single stage vs Multistage, Horizontal vs Vertical) and the number of pump

units required to achieve the desired pumping duty. API 610 - VS6 (Vertical Barrel) type pump is a taken

as a default selection when the system NPSHA is too low to meet the required NPSH margin by using a

Horizontal Pump.

As a common practice, basic pump type and number of pumps required is defined by the Process

Engineer in the Concept Optimization stage and/or FEED stage. However, it is a fact that most of the

Process Engineers do not have the experience and skill to identify and foresee the mechanical design

and sizing limitations for the rotating equipment. On the other hand, people involved in defining

mechanical design at the FEED stage tend to blindly follow the process duty parameters including the

pump type defined in the process deliverables. One of the reasons for such a practice is that the

Mechanical Engineer assigned by the FEED Contractor is often a general Mechanical Engineer with

limited or no expertise in Rotating Equipment design and selection. On the other hand, some FEED

Contractors even do not involve Mechanical / Rotating Specialist in the FEED development and the

whole system design is solely driven by the Vendors/Suppliers who have their own vested interests and

they influence the FEED Contractor to knit the design which suits their particular equipment/package

design and manufacturing range.

Though deficiencies in the FEED are identified at the EPC stage; incorporating changes at the EPC stage is

not an easy task and most of the times it is not even practical. As a result, the FEED transforms into the

EPC detail design without changes. The ultimate looser is the plant owner who is deprived of his

precious money in terms of spending unjustified CAPEX and/or left with a never ending episode of

operational problems inherited from an improper equipment selection at the FEED stage.

This technical paper provides a highlight on two major type of pumping systems with examples taken

from the real life projects where VS6 type pumps are commonly used. For each example, a brief

description is also provided to elaborate the pump type selection.

Finally, two case histories are described where VS6 type pumps were selected. These case histories are

the typical examples for VS6 type pump application where the pump selection was not appropriate and

it finally resulted into future operational troubles or caused expenditure of excessive CAPEX which could

otherwise have been saved with a technically wiser pump selection. The Project name, Contractors and

Company names are withheld so as to maintain a neutral position avoiding any stated or implied

criticism.

Flow

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Typical Applications for Vertical Barrel (VS6) Type Pumps:

There are two types of pumping systems where VS6 pumps are employed due to low NPSHA.

Type-1: Booster Pump (Vertical - VS6) + Main Pump (Horizontal – OH or BB type)

This approach is used when the either “Flow Rate” or “differential head” is too large to be handled by

VS6 pump alone (very high head or very high flow or a combination of both). In such cases, VS6 pump is

used as upstream “Booster” Pump to pressurize suction header for another set of downstream Horizontal

Pump used as “Main” Pump. A typical sketch for such pumping system is shown in Figure-1 below.

Figure-1: Typical pumping system with Booster & Main Pump combination

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An example for a pumping system shown in figure-1 is described as below:

Description Unit Specification

BOOSTER PUMPS

Plant Location - Ras Laffan Qatar

Service - HC Condensate Booster Pumps

Rated Flow ( Q ) m3/h 2500

Differential Head ( H ) m 40

NPSHA m 1.1

Motor Rating kW 300

Operating Speed rpm 970

Pump Type - API 610 – VS6 type

Number of Pumps - 3 x 50% (2 oper + 1 stndby)

Pump Size m Pump Length: 11.7 (top to bottom) Pump Can Diameter: 1.3

Figure-2: Booster Pump Installation Photo

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MAIN - LOADING PUMPS


Service - HC Condensate Loading Pumps



NPSHA m 9.2

Motor Rating kW 750


Pump Type - API 610 – BB1 type


Skid Size m Length: 8.4 Width: 2.5

Figure-3: Loading Pump Ready for Installation Photo

Discussion on Pump Selection:

Total differential head in the above pumping system (Booster Pump: 40m + Loading Pump: 98m = 138m)

was not very much high and could be easily be produced by a multistage VS6 type pump alone without

requiring a downstream “Loading Pump”. However, the required flow rate “2500 m3/h” was too high to

be handled by a VS6 type pump in combination with 138m differential head. Therefore, a combination of

a “Booster” and “Main - Loading” pump was used. The VS6 type Booster Pump was selected such that it

produced enough differential head (40m) to meet the NPSHA requirement of the downstream “Loading”

Pump and also differential head required for “Inter-tank” transfer etc. The “Loading” pump was a BB1

type pump with double suction impeller (2 suction eyes per impeller) to meet the high flow (2500m3/h)

and low differential head (98m).

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Type-2: Using Vertical - VS6 Pump Alone

This approach is used when the system NPSHA is too low and “Flow Rate” and “differential head”

combination can be covered by a VS6 pump alone with the available pump size in the required range of

(Q-H) and which does not un-realistically increase the pump size to make it operationally un-reliable in

terms of mechanical design. A typical sketch for such pumping system is shown in Figure-4 below.

Figure-4: Typical pumping system with VS6 Pump without Booster/Main Combination

A good example for a pumping system shown in figure-4 is described as below:



Service - Off-Spec Condensate Recycle Pumps



NPSHA m 0

Motor Rating kW 75





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Figure-5: Off-Spec Condensate Recycle Pump Installation Photo

Discussion on Pump type Selection:

Total differential head in the above pumping system (530m) was not very high and could be easily be

produced by a multistage VS6 type pump alone without requiring a combination of an upstream Booster

with a downstream Main pump. Also, the flow rate (30m3/h) was not too high to be handled by a single

VS6 type pump in combination with 530m differential head. Therefore, a single VS6 type pump was

selected to meet the required pumping duty.

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CASE HISTORIES FOR VS6 TYPE PUMP SELECTION

Following are the few cases from real world projects which serve as a good example to illustrate VS6

type pump selection in various applications as preceding section of this paper.

CASE HISTORY - 1: NGL SHIPPING PUMPS

(2150 MM SCF/day GAS TREATMENT PLANT)

This case history presents pump selection for NGL Shipping Pumps where system NPSHA was too low to

apply Horizontal Pumps and hence VS6 type pumps were required in the FEED design. However, the

operating conditions and number of pump units specified in the FEED data sheets were practically

unrealistic to apply a VS6 pump as 3 x 50%. The required flow rate (605 m3/h) and differential head

(1100m) combination was outside the limits of VS6 type pump size available in the market. Taking

individually, it was the rated flow which was not fitting into the situation to get 3 x 50% VS6 type pumps.


Service / Location - NGL Shipping Pumps / UAE

Rated Flow ( Q ) m3/h 605 ( = 1210 / 2 )

Differential Head ( H ) m Day 1: 875 (Future: 1100)

NPSHA m 3.9



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In order to suit pump selection on realistic basis, the required flow per each pump was reduced and the

number of pump units was increased from 3 x 50% (2 operating + 1 standby) to 4 x 33.3% (3 operating +

1 standby).


Service / Location - NGL Shipping Pumps / UAE

Rated Flow ( Q ) m3/h 403.4 ( = 1210 / 3 )

Differential Head ( H ) m Day 1: 875 (Future: 1100)

NPSHA m 3.9

Motor Rating kW 1250



Number of Pumps - 4 x 33.3% (3 oper + 1 stndby)


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Discussion on Pump type Selection:

Though the EPC Contractor was finally able to source the NGL Shipping Pumps with a change in required

flow per each pump, the differential head (1100m) was on a very high side for a single VS6 type pump

application. The design was still not very good and not something recommendable (huge pump with high

operating speed, a vertical pump operating under very high differential pressure and hence very high

axial thrust load with huge thrust bearing requiring force feed lubrication system); however there was no

choice other than to proceed with the 4 x 33.3% design using VS6 pumps alone to avoid the huge impact

on the plant layout, cost & schedule impact in case of suggesting a booster & main pump combination to

the client.

The best FEED design should have been a combination of an upstream VS6 type “Booster” pump

designed to provide small to moderate differential head with a downstream Horizontal type most

probably a multistage BB3 or BB5 type pump .In this particular application, a combination of a booster

and main pump could have worked even for 3 x 50% selection basis arriving at the pump design pretty

much reliable than the design actually implemented with VS6 type pumps alone. These NGL Shipping

Pumps have already been commissioned and under normal operation. As per feedback from the site and

plant operations, the pumps suffered from several problems including high vibration at the

commissioning stage and still have problem of high vibration during the regular operation phase.

Figure-5: NGL Shipping Pump at Test Rig in Flowserve Shop (Vienna - Austria)

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Fiure-6: NGL Shipping Pump Installation View Showing Pump and LO System

CASE HISTORY – 2: CRUDE OIL PUMPS

(13,200 BPD OIL FIELD DEVELOPMENT PROJECT)

This case history presents pump selection for Crude Oil Pumps where system NPSHA was too low to

apply Horizontal Pumps and hence a combination of upstream VS6 type Booster pump with a

downstream “Main – Export” pumps was selected in the FEED.


BOOSTER PUMPS

Plant Location - Algeria

Service - Crude Oil Booster Pumps


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Differential Head ( H ) m 105.3

NPSHA m 1

Motor Rating kW 30



EXPORT PUMPS

Plant Location - Algeria

Service - Crude Oil Booster Pumps



NPSHA m 84

Motor Rating kW 160

Pump Type - API 610 – OH2 type


Discussion on Pump Selection:

The selection of booster and main line combination specified in the FEED was not the best selection. The

combination of required flow rate (70 m3/h) and differential head (Booster: 105.3m + Export: 270m =

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375.3 m) was well within the application range of a single VS6 type pump size easily available in the

market. Therefore, by using a good design approach during the FEED, this pumping system could have

been simplified by using VS6 pumps alone without using a booster and main pump combination.

CONCLUDING REMARKS

Rotating equipment selection must be carefully reviewed and approved at the early stage of the project

including Concept and FEED. It must be ensured that the equipment/system design defined in the FEED

is practically viable and at least three competent and reputed Suppliers/Manufacturers are able to

supply the equipment based on those design specifications. Once the FEED is frozen and project enters

into the EPC stage, it becomes very difficult to incorporate any changes in the FEED. The EPC Contractors

are bounded by the lump sum nature of the contracts which does not allow them to raise any changes

which may negatively impact the timely and economically favorable completion of the project. While on

the other hand, most of the Client Companies do not welcome any changes in the FEED considering it as

a breach of the original design prepared by their highly trusted Engineering Design Consultant.

About the author

Muhammad Imran is Mechanical Rotating Equipment Engineer presently working with LUKOIL International Services B.V based in Dubai. He has 15 years of professional experience in Oil & Gas and Petrochemical projects including EPC detail engineering, PRE-FEED, FEED and hands-on experience in the Pre-commissioning, Commissioning, Startup and initial operations. Before joining LUKOIL, he worked for WorleyParsons Qatar as Lead Mechanical Design Engineer for Rotating Equipment and Packages. He can be reached at “[email protected]”.

LOW NPSH & CASES OF VERTICAL BARREL (VS6) TYPE PUMP SELECTION

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