Latex production
Variable Speed Pumps reduce valve wear to a minimum

Steve Minett, PhD & Feld Kenwick

A boiler plant in north-west England provides steam to an adjacent PVC manufacturer. The use of variable speed pumps, controlled by microprocessor units, has, over the last three years, provided significant benefits for their combined operations. The constant start-up and shut-down pattern of plant usage that can cause wear on valves and increase maintenance is reduced this to a minimum by the soft start/stop cycles.

The production of latex involves triggering an exothermic reaction by heating autoclaves containing the reactive chemicals. Once the reaction has been started, the external supply of heat is no longer required. This process produces a saw tooth demand pattern which requires highly responsive control of the system for minimum energy consumption. BP Energy - part of the British Petroleum Group - supplies steam and de-mineralised water to European Vinyl Corporation’s PVC9 plant in Thornton-Cleveleys, near Blackpool, on the north-west coast of England. This plant produces about 45 to 50,000 tonnes of latex per year. The fluctuating demand for boiler feed water is met by three ITT Industries’ Lowara SV 1610 N110T variable speed pumps controlled by Hydrovar microprocessor units.

The latex plant
The heart of the latex plant consists of six autoclaves surrounded by steam-heated coils, suitably jacketed. Latex production is a batch process which requires the vessels to be operated in rotation, thus producing the sharply fluctuating steam demand. Overlaid on this pattern, are steam requirements for other processes around the factory such as raw materials drying, water heating, etc. The process also requires PVC powder to be dried and this is carried out by using the exhaust from a gas turbine generating 5 MW of electricity for plant operation. About 4 MW is exported to PVC9 while BP Energy uses the remainder for its own plant operation. There are stand-by gas burners for use when the turbine is shut down for overhaul and routine maintenance.

Water supply
Raw water is stored in a buffer tank of 20 tonnes capacity from where it is pumped to the demineralisation plant by two ITT Industries SV 6004 110T. There are two demineralization streams, each having a cation resin bed for removal of calcium, magnesium and similar salts, and an anion resin bed for removal of silica and similar impurities. The streams alternate on duty while the resin beds are being regenerated.
The flow through each stream averages 7 t/h although each stream has a maximum capacity of 27 t/h. This represents 8,400 m3/month, of which 5,200 m3 goes direct to the latex plant for use in emulsification of the product while the remainder goes to the hotwell for boiler feed. After treatment, the conductance of the water, a measure of purity, is below 5 micro Siemens (mS).

Boiler plant
The boiler plant was built by BP Energy in 1998 to take over supply after the power station, which was supplying steam to PVC9, was decommissioned in 1999. There are two gas-fired shell boilers, each with a nominal steam-raising capacity of 15 t/h, normally producing 6 t/h. The usual operating regime is to have one on duty and one on hot standby. Plant technician Paul Crumpsty: “The normal operating pressure of the duty boiler is 10.3 bar while the standby unit is run at 6 - 9 bar. The pumps have to work against this head of pressure but they are capable of delivering against higher heads if required. It is only a matter of minutes to bring the standby boiler up to pressure,” he said. “The Hydrovar unit enables us to make best use of energy with all the demand changes. We have to follow the production pattern in PVC9, twenty four hours a day, seven days a week and the penalties would be serious if we did not have the flexibility of the system.”

The Hydrovar System - the Basic principle
The heart of the energy-saving principle of variable-speed pumps is the basic hydrodynamic law that the power consumed by centrifugal pumps varies as the cube of impeller speed. So if pump speed is reduced by one unit, energy consumption is reduced by four units. Hydrovar’s intelligent controllers improve on this already significant saving by minimising friction losses associated with the fluid flow.
At low fluid flow speeds the head lost by friction is proportional to the velocity but at higher speeds, the head loss is proportional to the square of the velocity. The Hydrovar system maintains a plant curve which reduces pump speed to reduce fluid flow wherever possible to below the critical speed where linear losses become square law losses; this additional function increases energy savings by some 20 per cent. In addition, the units have a patented cut-out which switches off the pump when the flow is zero.

An analogy
Manfred Sacher, is the product manager for the Hydrovar speed control system at ITT Vogel just outside Vienna, where Hydrovar was developed. Manfred likes to use a car analogy. He suggests that constant speed pumps operate very much as if a person was driving a car with one foot constantly on the accelerator and using the brakes to control the car’s speed and to stop. “The Hydrovar system,” he says, “is like a car with an automatic gearbox; and, extending the analogy, even like an automatic car with cruise control.”
He explains that with a constant speed pump there are basically three flow control methods. Firstly, the flow can be reduced mechanically by throttling the discharge or returning excess flow to the suction side of the pump. Secondly, a by-pass system can reduce the flow to the pump; apart from wasting energy, this system can induce cavitation in the impeller causing additional wear. Thirdly, hydraulic accumulators can be used to absorb excess flow and store it under pressure. When the pump output drops below demand, the accumulator can be used to bring the flow up to the desired level. Accumulators are expensive, take up space, have limited capacity and seldom produce a smooth and constant flow.

Frequency conversion
The speed of a simple induction motor depends on the frequency of the AC power supplying it. In most of Europe mains supply is at 50 Hz (cycles per second) and in the United States 60 Hz, so motors connected directly to the mains turn at multiples of these figures depending on how the motor is wound. To alter the frequency of the motor supply and thus regulate pump speed, the Hydrovar system rectifies the mains supply to DC and then inverts it under command from the controller to provide the frequency required to match pump demand.
Input to the frequency controller comes from pressure and flow sensors; these inputs are integrated with the operator’s programme to provide a fully flexible operating regime. The system provides not only economical pumping but also incorporates safety features and provides solutions to special requirements.

Packaged system
The key to this level of automation and flexibility in the Hydrovar, Manfred explains, is the inclusion of a micro processor actually in the Hydrovar unit. Pressure and flow sensors are also attached to the actual pump. The Hydrovar concept therefore is to have all the equipment necessary for a variable speed control system mounted on the pump itself. One of the many advantages to this ‘pump-mounted’ solution is that air from the motor cooling fan can be used to cool the electronics. Exceptionally, for pumps operating in hostile environments, the control gear is available as a wall mounted unit. Since the Hydrovar unit is so self-contained, it can be moved from one pump motor to another and can also be retrofitted to existing pumps.
Manfred comments that several competitors offer frequency inverters but without micro processors which must be supplied separately. This means that they have to be connected to remote control units, thus introducing additional installation costs. They also have to be programmed before the variable speed system can be used. Many inverters are limited to a maximum rating of 7.5 kW whereas the Hydrovar is rated up to 22 kW.

Multi-pump systems
Another unique feature of the Hydrovar is its application in multi-pump systems. With the plumb in and pump principle, the only extra installation work is for each pump in the system to be connected with an interface cable to its neighbour and for the pumps to be named so that the micro processors can identify each pump. Hydrovar can be retrofitted to existing multi-pump systems and it can include the friction loss compensation system.
The multi-pump system is available for a maximum of four units and it ensures a step-less transition between each pump coming to maximum speed and the next one starting. Uniquely to Hydrovar, built-in redundancy ensures that if any component on any one pump fails, for example sensors, inverter or micro processor, the other three can maintain system pressure and avoid breakdown.

Advanced features
Another feature of the Hydrovar is what Manfred calls the inverse function. He explains that, “normally speed control systems depend on signals from sensors located on the discharge side of the pump. Hydrovar can also be programmed to respond to signals from a pick-up on the inlet side of the pump.” An example of where the inverse function might be used is in cooling water systems where the temperature of the incoming water would be a factor in the control of pump speed. This would enable flow through the heat exchanger to be increased to augment the cooling effect.
Hydrovar can be programmed with two different pressure levels to accommodate different operating regimes, for example, daytime and night time, with a timer to switch over to each pre-determined setting. The system can also use different variables, for example pressure, fluid level or fluid flow. The pump could be programmed to operate at a constant pressure up to a point determined by flow or water level and then its speed would be determined by the second variable.
Manfred emphasises that Hydrovar is not simply a pump speed control system. He likes to call it the independent brain on a pump. It can enable operators to get optimal pump performance without having to acquire a quantity of additional equipment

The Hydrovar experience
“We have had three years experience with Hydrovar and the advantages are crucial,” says Paul Crumpsty, at the BP Energy plant. “The boiler feed from the hotwell is at 84 deg C and constant speed pumps can cavitate under these conditions, which causes impeller erosion. This reduces efficiency and increases maintenance costs. With constant speed pumps the water can boil in the pump and we have to ‘leak-off’ to stop it. This is noisy and consumes a great deal of power. The constant start-up and shut-down pattern of plant usage can cause wear on valves and increase maintenance but Hydrovar’s soft start/stop reduces this to a minimum.”
Shift technician Tony Byrne highlights the energy saving compared with constant speed pumps. His opinion is borne out by a recent Lowara study of the plant: before the Hydrovar was fitted power consumption was about 85,000 kWh/year and after it was fitted this was reduced to 80,000 kWh. <<