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Balancing pump efficiency with=20 NPSH19 August=20 2008Selecting a pump is a balance of many factors, including = the=20 volumes and contents to be pumped, the efficiency of the pumps and = how=20 frequently the pump will be run.
Oliver Brigginshaw, managing director of Amarinth, a = company=20 specialising in the design, application and manufacture of pumps = and=20 associated equipment, explains NPSH and how pumps can be cost = effectively=20 engineered to operate in a low NPSH environment. Cavitation = occurs=20 when the pump cannot get enough liquid and the resulting reduction = in=20 pressure causes liquid to vaporise and form bubbles. These bubbles = can=20 grow dramatically and choke an inlet, further reducing the flow of = liquid=20 and the performance of the pump. In addition, these bubbles can = implode=20 with tremendous force, literally tearing away at the metal. The = resulting=20 increase in vibration and noise can lead to premature component=20 replacement and in some cases complete failure of = impellers. To=20 avoid this catastrophic situation, the pump manufacturer should = always ask=20 you for the Net Positive Suction Head available at the pump - = NPSH(A) -=20 and ensure this exceeds that required by the pump to operate = without=20 cavitation occurring =E2=80=93 what is known as NPSH(R). See Fig=20 1. For a given NPSH(A), the pump manufacturer will = provide a=20 pump with an NPSH(R) less than NPSH(A) by some 0.5m, though if the = accuracy of the data is circumspect (such as pumping a cocktail of = fluids), then it may be better to increase this=20 difference. Generally pump manufacturers design their pumps = to work=20 at maximum efficiency and hence the lowest running costs. = Efficiency is=20 related to pump speed and so the speed is fixed to deliver the = maximum=20 efficiency for a given pump. These parameters dictate the NPSH(R) = for any=20 given pump. Important as efficiency is in a pump though, it = is not=20 the only criteria when selecting the most cost-effective pump for = the job.=20 Achieving the necessary static head to run a pump at its optimum=20 efficiency may be either impossible to achieve or could involve = costly=20 changes to plant structure, pipe work and associated equipment. If = faced=20 with this situation, what are the alternatives? This means that if you require a low NPSH pump = then=20 you need to turn to one of the specialist pump companies such as = Amarinth.=20 Using sophisticated tools and techniques they can = =E2=80=98tune=E2=80=99 existing=20 designs and re-engineer key components to produce pumps capable of = delivering the required performance within the NPSH constraints of = the=20 plant. They also have comprehensive test facilities, as finding = the point=20 of cavitation reliably is a crucial, but largely empirical,=20 process. The resulting pump will be one that can deliver = the=20 specified performance within the suction static head constraints = at an=20 efficiency approaching that of a standard pump. The cost of = undertaking=20 these modifications is of course not cheap, but it is almost = always=20 outweighed by the cost or sheer impracticability of modifying the = plant=20 structure. Initially a = =E2=80=98vertical in-line=20 pump=E2=80=99 was considered but this was quickly ruled out as it = required=20 significant headroom to remove the motors during maintenance, = which was=20 not available. Additional pipework before and after the pump would = also=20 have been required, expanding the footprint beyond the space=20 available. To achieve the dimensional space constraints = within the=20 tight contract deadline, a completely new pump design was not = feasible and=20 so Amarinth proposed a hybrid design. This was based on its proven = =E2=80=98C=E2=80=99=20 series heavy duty ISO 5199 chemical pump. This = =E2=80=98horizontal=E2=80=99 pump would fit=20 the space constraints and was already suited to operating in low = NPSH=20 environments. Up-rated API 610 bearing brackets were = designed and=20 incorporated which would allow the pump to meet the demands of the = application, both in terms of the space available and the NPSH = constraints=20 of the vessel, thus achieving Wood Group=E2=80=99s requirements of = low=20 maintenance, high reliability pumps that required low NPSH(R). The = alternative would have been to redesign the system and pipework or = at=20 worst build new decks to deliver the NPSH (A) that a standard high = efficiency pump would require for normal = operation. |
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