Hervé Baron, who initiated and coordinated this article, is the author of The Oil & Gas Engineering Guide (editions Technip)
This book provides the reader with:
a comprehensive description of engineering activities carried out on oil & gas projects,
a description of the work of each engineering discipline, including illustrations of all common documents,
an overall view of the plant design sequence and schedule,
practical tools to manage and control engineering activities.
This book is designed to serve as a map to anyone involved with engineering activities.
It enables the reader to get immediately oriented in any engineering development, to know which are the critical areas to monitor and the proven methods to apply.
It will fulfill the needs of anyone wishing to improve engineering and project execution.
This article aims to equip non-specialists with all they need to know or understand about pumps in oil, gas, and—collectively called--hydrocarbon processing services. The emphasis is on the key technical features and considerations when specifying a pump and checking that technical issues and realistic economic choices remain in balance. High asset reliability must be obtained throughout.
This article covers both centrifugal and positive displacement (PD) pumps. PD pumps include reciprocating and rotary (screw, vane, lobe, gear) types. For each topic, criteria relating to both centrifugal and PD pumps are reviewed side by side, so as to identify their relevant differences and similarities.
Fig. 1: A single-stage back pull-out pump
Fig. 1 shows a centrifugal (overhung) pump. This is the type of pump most frequently used in the Hydrocarbon Processing Industry (HPI). It is safe to assume that close to 90% of the pumps in the HPI are of this type.
Inquiry documents issued for pumps:
Inquiry documents are typically issued by an Engineering Company (often labelled EPC, which stands for Engineering Procurement and Construction provider. The EPC compiles and issues these inquiry packages on behalf of the owner-operator or ultimate purchaser. The bidder replies with a cost quote and detailed description of the equipment offered.
More specifically, the EPC later issues to the selected bidder a set of pump purchase documents. These would become a binding specification of deliverables; for process pumps, the set of purchase documents usually consists of the Material Requisition and attachments, as follows:
One data sheet for each pump
A general specification, describing the Purchaser/Client’s choice among the various choices proposed in applicable American Petroleum Industry (API) standards, together with the Client’s additions, substitutions and/or modifications to API requirements, if any. On bulleted items listed in an API standard, the EPC must indicate which items to delete or to invoke. Not making this choice usually results in the vendor supplying least cost options
An Inspection and Test Plan, or requirements related to inspections and tests (see dedicated section) and owner-purchaser’s involvement
A vendor documents list and schedule (the deliverables)
The required contents of the Material Requisition (MR), which includes site atmospheric conditions, power supply and utilities available, restricted sub-vendors list, etc., are the subject of this post.
It is useful to include in the MR a ”special notes” section, highlighting the specific, stringent and reliability-imparting requirements of the job. These requirements are indicated in the inquiry documents, e.g., on the data sheets, and reference should be made to these documents. However, it will be effective to group them in a dedicated section to emphasize their importance.
Centrifugal or reciprocating pumps?
Centrifugal pumps should be selected wherever possible because they cost less and are more reliable. The only exceptions for not using a centrifugal pump but a positive displacement pump instead, are:
very low flowrate, less than 3.5m^3 per hour
very precise (low) flowrate
very high head, over 300 bar
very viscous fluid, above 430 cSt, due to significant reduction in efficiency.
Centrifugal pump speed (rpm) has traditionally been limited to 3,000 rpm (at 50Hz) / 3,600 rpm (at 60 Hz) due to motor design. Higher speed pumps require a gear box or a Variable Frequency Drive. These high-speed centrifugal pumps can be an option when high energy must be imparted to the fluid, such as for high pressure water reinjection, etc.
The process engineer usually specifies the type of pump on the Process Data Sheet. How does he/she determine whether the flow + head combination is within the capability of a centrifugal pump? Fig. 2 depicts the head vs. flow chart, and associated guidelines, to help the specifier fill out the process data sheet.
Fig. 2: Pump selection chart
1. Selection of pump type should conform to Fig. 2, which shows the approximate upper limits of differential pressure and flow rate for the various classes of pumps. Rotary type pumps include screw, vane, gear and lobe pumps.
2. Where a chart allows more than one class of pump, the selection sequence should favor centrifugal, rotary, then reciprocating, depending on factors shown below.
Flow regulation: Flow in centrifugal pumps varies widely with system resistance. In positive displacement pumps (rotary pumps and reciprocating pumps), the mean flow remains practically constant while the pumping energy will vary depends on system resistance. If the service requires high flow stability, a positive displacement pump is the appropriate choice.
Liquid viscosity: Centrifugal pump performance deteriorates rapidly with increasing viscosity, and a PD pump is usually the better choice whenever the liquid viscosity exceeds 500 SSU (110 cSt).
Energy consumption: Displacement pumps are more efficient than the equivalent centrifugal pump, particularly those near the centrifugal pump upper limit, although with higher capital and usually higher maintenance costs. Consider the choice of PD pump when the energy cost is high.
3. Within the PD pump group, where both rotary and reciprocating pumps are allowed, the choice is rotary, then reciprocating, subject to two general limitations:
Low viscosity: Performance of rotary pumps falls off when liquid viscosity decreases because of an increase of internal leakage of pump. If the pumped liquid has low viscosity, lower than approximately 100 SSU (22 cSt), or low lubricity and the differential pressure is high, a reciprocating pump is the more appropriate selection.
Abrasives: Rotary pumps have a low tolerance of abrasive solids in the pumped liquid because of their small internal clearances. Reciprocating pumps are preferred when liquids contain abrasive solids.
However, pump manufacturers have been extending continuously the range of centrifugal pumps so that it is best to consult them.
Positive displacement pumps include reciprocating pumps, which have pistons and plungers. There are also controlled volume pumps, which have adjustable piston stroke lengths, gear pumps of internal or outer tooth construction, vane, screw and rotary progressive cavity types.