Devil in the detail 06 August 2014
Pump designs may not be subject to radical change these days, but the sheer variety of types and the challenges of pumping applications make this a specialist sector. Brian Tinham reports
Anyone doubting that there's more to pumps than shape, size, type and power, needs to visit the relevant Wikipedia web page. Not only are they also the subject of several competing operating principles, but there are significant functional variants within each – and that's before you to get to the seal types, materials of construction, etc. And, while there is bound to be some overlap in capabilities, and the usual pros and cons between alternative technologies, the point is it's worth updating your knowledge, if you want to select the best, most efficient and cost-effective pumps for your applications.
Obvious, maybe, but how many of us actually do so? Yet just consider positive displacement (PD) types. Pumps that fall into this category alone include: rotary (internal gear, screw, shuttle block, flexible or sliding vane, circumferential piston, flexible impeller, helical twisted and liquid ring vacuum type); reciprocating (including piston – simplex, duplex, triplex and quad – and hydraulic diaphragm pumps); and linear (for example, rope and chain pumps). Some of those classifications may seem arcane: for rotary PD types, you should, for example, be more familiar with rotary lobe, progressive cavity (helical rotor), rotary gear, screw-type, rotary vane and probably also peristaltic pump types.
Then there are centrifugal types – the classic motor-driven impeller and volute, otherwise generally known as roto-dynamic and velocity types, which include radial-, axial- and mixed-flow versions, as well as multi-phase helico-axial types. Others include: impulse pumps (such as hydraulic ram, pulser and airlift types); gravity pumps; and valveless pumps (such as those typically designed for biomedical engineering applications).
Pump pros and cons
If you're looking for distinctions, experts helpfully inform us that positive displacement pumps theoretically produce a constant flow for a given rpm, irrespective of discharge pressure – unlike centrifugal or roto-dynamic types – although slight increases in internal leakage as pressure builds do have an impact.
On the other hand, PD pumps cannot be operated against closed valves on the discharge side, because they don't have centrifugal pumps' shut-off head – so pressure increases until the line bursts and/or the pump is wrecked. Hence manufacturers' preference for internal relief or safety valves on the discharge side.
Meanwhile, rotary vane pumps – which move fluid by creating a vacuum on the inlet side – are not only efficient, given the correct head parameters, but also remove air from lines, eliminating any bleed requirements. That said, manufacturing tolerances are obviously critical and these pumps have to be operated on the slow side. At high speeds, fluids cause erosion, eventually enlarging clearances and, in turn, damaging efficiency.
But we're scratching the surface. Mike Eason, who started his career at Sulzer as a design engineer and has spent the last 20 years on positive displacement pumps (diaphragm and piston), latterly with Thorite, suggests that there's a lot more to this than generalisations. For example, he observes that, while it is easier to choose positive displacement pumps than centrifugal types, where efficiency demands operation at certain points on the head, power and NPSH (net positive suction head) curves, there are other practical considerations.
"Think about product temperature and viscosity, not just rate and head," suggests Eason. "We had one application where the material to be transferred had a low measured viscosity, yet high-speed pumping turned it into a blancmange. In fact, it was quite rubbery and had to be treated as a solid, using air-operated piston pumps."
He concedes that air-operated PD pumps are not as efficient as their electrically-driven counterparts (although they can be as clean with today's non-lubricated air motors), but makes the point that they have their advantages, too. "If the product you're pumping changes in temperature or viscosity, for example, air-operated pumps can accommodate that, whereas electric motors would slow down or potentially even burn out."
And he reminds us that difficult fluids – such as mastics, pharmaceutical creams, petroleum jellies, cottage cheese and PVA – have to be pumped slowly, with the right inlet conditions, using equipment that can be primed.
"You can't treat these materials like water and transfer them in bulk, using centrifugal pumps running at typically 1,450rpm. You have to think about the product and what will happen to it, and then choose pumping systems and production rates accordingly."
At the other extreme, Oliver Brigginshaw, managing director of specialist centrifugal pumps manufacturer Amarinth, points to two API 610 OH2 units recently delivered to Ineos for the flare knock-out system at its Grangemouth refinery.
The pumps, which have Plan 53B seal support systems, were for its mercaptan oxidation (Merox) process and will be used to remove condensate that collects in the flare knock-out drums during refining.
These had to be ultra reliable, because they will only be operated infrequently, sometimes remaining dormant for weeks. Also, they had to be resistant to the often hostile Scottish coastal saline environment. Hence, Brigginshaw explains that, quite apart from the raw pumping duty, his team designed these pumps to withstand temperatures of
-20ºC, while surfaces were painted to offshore standards. He also says that oilers were incorporated so that water in the bearing oil can be identified and removed.
And to ensure reliable operation on demand, Amarinth also worked with Ineos to develop a maintenance schedule that minimises the risk of damage due to periods of inactivity – particularly around preventing seal faces from sticking.
Clearly, the devil is in the detail. Precise design can make or break a pump's success in an application, as flexible pouch packaging machines firm Thimonnier found. It chose MasoSine EC25 pumps, from Watson Marlow, for a new unit designed to convey béchamel sauce to dosing nozzles, primarily because of their non-shearing and hygienic characteristics.
MasoSine pumps do not have valves, their chambers are sealed and they are compact. The sinusoidal rotor creates four moving chambers that gently transport the fluid from suction to discharge without any variations in volume. Also, the gate created prevents any fluid from passing back from discharge side to suction.
"Two EC25 pump heads have been integrated into our packaging machine," confirms Frédéric Roumanet, design office manager at Thimonnier. "They gently convey the sauce for dosing into the pouches," he continues, adding that CIP (cleaning in place) has also been accommodated by taking advantage of the positioning flexibility allowed for the pump heads, which eliminates bacteria retention.
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