Water is the most popular heat transfer agent used in industry. It is readily available, cheap and above all not toxic. Up to +100 °C it can be handled easily with any pump and shaft seal. At higher temperatures the heat transfer systems must be pressurised in order to avoid vaporisation. The vapour pressure rises rapidly, which stresses circulation pumps and sealing systems tremendously.
Ralph Schommer
Dickow Pumpen offers a wide range of hot water pumps for temperatures up to +280 °C and design pressures up to 80 bar. For temperatures up to +190 °C and 22 bar, pumps based on the chemical standard concept (medium-duty pumps acc. to DIN) with ductile iron casings are sufficient. Applications up to +238 °C and 45 bar are covered by the NHL series, which is specially designed for the high pressures of such plants. The casings are made of high temperature-resistant carbon steel and optional centreline mounting is available. Extended casing gasket surfaces and heat-resistant, expansion-type casing bolts prevent leakage of the hot water.
Extremely high temperatures of 280 °C and system pressures up to 80 bar can be handled with the high-pressure HPL/HPR models. The casings of this pump type have been completely revised: they now have better efficiencies and can generate higher differential pressures. Six casing sizes are available instead of four, with flows up to 280 m3/h and heads up to 62 m. These casings are likewise made of high temperature-resistant carbon steel and centreline mounting is standard. The high suction pressures additionally require heavy-duty ball bearings in order to achieve a sufficient bearing lifetime.
Shaft sealing systems
Owing to the high vapour pressures, standard mechanical seals cannot be used at higher temperatures. The easiest and cheapest solution is a cooled mechanical seal. The required cooling water, however, generates additional costs for water pipes and water treatment (max. operating temperature: +238 °C).
Another possibility is to opt for a standard mechanical seal with a pumping device and an external air cooler. The cooler, which is a new development for this application, needs no cooling water and is self-venting (max. operating temperature: +210 °C).
For temperatures up to +280 °C, high-pressure mechanical seals with an external, water-cooled heat exchanger (API Plan 23) are essential.
Sealless pumps with a magnetic coupling can be used as an alternative to standard sealed pump types. The termination of the main wear part (the mechanical seal) leads to a much longer mean time between fail-ures and therefore higher availability. Hot water spill due to mechanical seal failures is also ruled out. The current operating limit for magnetic couplings in hot water applications is approximately 43 bar at +250 °C.
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