Chemical Technology August 2015

PUMPS AND VALVES

users specify a bypass line for unloaded startup. Other designs incorporate mechanical suction valve unload- ers which are functional until the pump reaches 100 % speed. Starting against line pressure (such as in multiple pump applications where they must start against full line pressure), necessitates a driver with higher start- ing torque capability. Other options are reduced voltage starting or use of soft start devices such as fluid coupling. • G: Reciprocating pumps are unsuitable when pulsating flow can not be tolerated, especially if flow and pressure control instruments are involved. It should be recognised that pulsations can be damped but the equipment to achieve pulsation control can be expensive. Options to look at are rotary or centrifugal pumps because of their steady discharge pressure and linear flow. Many users stipulate piping pulsation analysis for reciprocating pumps with rated power exceeding 100 KW. • H: Reciprocating pumps should not be selected when leakage of pumped fluid must be avoided because of hazards, such as fire, toxicity, or loss of expensive fluid. A possible exception is reciprocating diaphragm pumps. • I: If ambient temperature is below the pour point of the pumped fluid, winterisation of pump casing (applies to spared pumps) and their associated auxiliaries (cooling water, minimum flow bypass, seal flush, and lube oil lines and oil filters, etc.) becomes essential. • J: Minimum 10 % head rise to shutoff is usually a require- ment for pumps in parallel operation. It is also important

limits stated in this guideline), where a vertical pump will result in significant savings in plot space, piping and foundation. It is worth mentioning here that selection of vertical pump impellers, either open, semi-open or closed, is governed by the pumping temperature, hydrau- lic balancing, efficiency, and fluid contaminants. • B: High speed vertical in-line pumps can be used instead of reciprocating pumps for low capacity, high differential pressure services. • C: Proportioning type injection pumps are suitable in acid, chemical and caustic services. Flow control of pumps can be achieved by adjusting the stroke. • D: Sealless pumps are ideal for handling hazardous (flammable, corrosive) and highly toxic liquids. Local and prevailing environmental regulations and risk avoidance standards may dictate the use of sealless pumps in such applications. Some pumpmanufacturers offer centrifugal and rotary pumps, both, in sealless configuration. • E: Consider rotary pumps when constant flow over a wide viscosity or flow proportional to rotational speed is required. Rotary screw or gear pumps are also preferred in high viscosity (~ 200 000 cSt) services. But slip in rotary pumps increases with reducing viscosity which in turn, affects capacity. Due to small internal running clear- ances, rotary pumps are not suitable for pumping fluids which contain suspended abrasive particles. • F: Capacity control of reciprocating pumps can be achieved by regulation of speed, stroke or by both. Some

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Chemical Technology • August 2015