The pressure is on

Getting the specification and installation of pumps right – and to some extent fans, too – still trips up plenty of engineers. And much the same applies to their ongoing maintenance in plants and factories right across the industry spectrum. Why? Mostly because of the sheer variety of applications, but also the range of equipment. Take centrifugal pumps, which are by far the most popular, given their relatively low cost, engineers' familiarity with them and, more recently, their ability to handle higher pressure heads. Even at a generic level, there are at least four types: rotary, overhung, submersible and horizontal self-priming. Why the range? Quite simply, because different applications demand different equipment attributes. But Kevin Lacey, of MRO (maintenance repair and overhaul) specialist Brammer, makes the point that making the right selection involves a great deal more than simply picking a broad pump type. He's right, of course. So what should you do? Well, the list of considerations is long, including: establishing the main process parameters (flow, pressure and temperature); understanding the fluid to be handled (density, viscosity, vapour pressure, solids in suspension, toxicity, volatility, aggressive media, etc); and taking into account the plant layout, available space, piping dimensions, etc. Lacey also recommends: determining elevation for suction and discharge points of vessels, relative to the pump centre line; calculating approximate friction losses and plotting system characteristic curves; and defining pump working parameters – capacity, head, suction and discharge pressures, including possible variations in pressure or temperature at different pumping conditions. And to those we might add: determining any exceptional start, stop or running conditions; and assessing available NPSH (net positive suction head). Only then are you ready to select the pump type, design, position, type of sealing, and whether or not cooling is required for the seal and bearings – and, of course, the drive type. That's generally (but not always) an electric motor, in which case Lacey recommends going for high-efficiency motors and VSDs (variable speed drives) to reduce energy consumption, but also to improve reliability and cut maintenance costs by reducing speed to match demand. The only caveat: don't fit VSDs where pumps are specified to operate under high head pressures, since the latter varies with speed, following the pump affinity laws. Other options to control pump speed include v-belts (particularly synchronous belts, which, once fitted and tensioned, do not require regular re-tensioning), especially where it is not possible to directly couple an online motor. Detailed selection Then, when it comes to selection, at an overarching level, he suggests: "The best option is to specify high-efficiency motors, which may cost more initially, but will reduce total ownership cost through lower energy consumption." Thereafter, it's about factoring in the above considerations. "The head depends on fluid density and viscosity, while flow rate determines pump capacity. Required flow is usually determined by material and energy balances, with a design margin typically up to 25%," he advises. This margin accounts for unexpected variations in properties and conditions, or to ensure that the plant meets performance criteria. Minimum flow protection is often added as continuous circulation. But we're not done yet. "Before selecting a pump, examine its performance curve, indicated by its head-flow rate or operating curve," warns Lacey. "The curve shows the pump's capacity, plotted against total developed head. It also shows the efficiency, required power input, and suction head requirements over a range of flow rates." Most important, though, pump curves indicate operating speed and impeller size per pump size and type – as well as the pump's BEP (best efficiency point). "Selecting efficient pumps and motors alone will not achieve cost effective and reliable operation," insists Lacey. "Pump characteristics must properly fit the system requirements throughout any process variations, ensuring operation as close as possible to the BEP." And that's not just for the obvious efficiency it brings, but also to avoid problems – the most serious being cavitation, which leads to fatigue and invariably early failure. Time for a swift audit? Amarinth wins TSB grant to develop custom pumps Specialist centrifugal pumps builder Amarinth has been awarded a £250,000 grant by the Technology Strategy Board to develop a design and rapid production process for highly energy-efficient pumps. The company intends to use the project to launch a new molten sulphur pump for the petrochemical industry. Amarinth has been working on this for some time, the goal being custom BEP (best efficiency point) impellers that can be produced on short lead times. Managing director Oliver Brigginshaw makes the point that the vast majority of centrifugal pumps throughout industry are 'best fit' selections taken from standard ranges – meaning that most do not run at their BEP, instead running with energy inefficiencies of up to 25%. In an ideal world, the impeller and casing of a pump would be designed and manufactured to each pump duty, he says, because the result would be a huge energy and CO2 saving over a pump's lifetime. Indeed, work by Amarinth to date suggests that optimised impellers could reduce annual CO2 emissions in the company's target oil and gas market by 17,000 tonnes by 2020 and 110,000 tonnes by 2050. Brigginshaw says Amarinth will use the TSB grant to further its development of both the pump design and production processes to deliver the most advanced and efficient bespoke centrifugal pumps at a commercially acceptable cost. That done, he indicates that the first application will be a pump for notoriously aggressive sulphur production processes that not only offers good energy efficiency characteristics, but also good maintainability and high reliability. "We have been working on this project for some time, initially with assistance from the Carbon Trust until the government curtailed that funding," comments Brigginshaw. "We are delighted that the Technology Strategy Board has now seen the enormous benefits that can be attained from our work." Energy-efficient fan systems The clock is ticking for inefficient fan systems. With the introduction of the Energy Related Product (ERP) directive, new industrial fans sold since January 2013 have had to meet mandatory system efficiency values that relate energy-in to airflow-out, according to type and size. And from January 2015, the regulations will be further tightened. Although the legislation is geared to improving total system efficiency – and indeed most manufacturers are taking the holistic view – the efficiency of the motor, impeller and transmission (unless it's direct-drive) are essentially the only variables available. Thereafter, it's about engineering teams correctly selecting and installing the right equipment. Frank Griffith, drives consultant engineer with ABB, concedes that, while induction motors exhibit losses in the 5-15% region (depending on size) and belt drives about 5-10%, the fan mechanism itself can be responsible for a stonking 10-50%. So, on the face of it, he agrees that selecting the right fan design has the biggest impact. "But it may not be feasible for any number of reasons – such as space or price... And the issue is similar when it comes to transmissions." Which leaves the motor. "At the moment, many fan systems use IE2 [high efficiency] motors, but manufacturers are increasingly asking for IE3 [premium] and even IE4 [super-efficient] motors," he observes. "IE3 motors are often adequate for systems with large and medium-sized motors. In smaller systems, where efficiency improvements are more difficult to achieve, then it may be necessary to look at other technologies [beyond induction motors], such as permanent magnet or switched reluctance motors." However, technicians specifying and installing fan systems also need to play their part. "Frequently, an oversized system is selected, as the additional cost seems small," comments Griffith. "But an improperly sized system will bring additional costs for energy and maintenance." And he notes that users are also responsible for providing ductwork that is sufficiently large and straight, to eliminate friction losses. A way forward, Griffith suggests, is for the person paying the energy bill to make the choice.