Shiyuan Water Plant plays an important role in delivering water for west area. Daily water delivery is 400000t. Its III phase project has 10 pumps. They are 550kW×3, 560kW×1, 400kW×1, 380kW×2, 355kW×2, 220kW×1. At present, variable frequency technology has been used widely in many fields. Most waterworks adopt variable frequency drive on motor. Its advantage is to heighten production safety and save energy.

In 2001, headquarter put 2 sets of Leader&Harvest VFD into operation in Shiyuan Water Plant. HARSVERT－A06/076 for 3# 550kW motor and 8# 560kW motor. Equipment has considerable energy saving effect after service of several years.

2.      Two operation mode analysis

Generally, make sample for pipe pressure by pressure transmitter. VFD converts pressure signal to electrical signal and sends it to PID regulator. PLC inside VFD makes comparison and operation with given pressure, then converts the result to frequency signal. VFD adjusts frequency according to coming frequency signal, thereby adjusting pump speed.

There are two modes according to variety of outlet pressure: variable pressure variable flow, constant pressure variable flow

2.1 Variable pressure variable flow system

Pressure transmitter is at end terminal of pipe. PID set value is water head of demand. System keeps pipe end pressure constant according to auto adjustment, and makes pipe character curve and static lift unchanging. But outlet pressure changes with supply flow changing according to pipe curve. In theory, it realizes “ pump can provide no matter how much system demands”. No excessive static lift created due to supply water reducing, energy saving is considerable. But it is just an ideal state, furthermore, only variable pump works solely. Because variable frequency equipment is expensive, most delivery water systems adopts mode of variable frequency pump and constant speed pump work in parallel.

Take example of mode: a variable frequency pump and a line frequency pump work in parallel. When flow reduces, lift needs to reduce. This can be done by variable frequency pump deceleration. In order to ensure parallel pumps work normally, lift of line frequency pump should lower. This can be done by increasing flow only, thereby water loss is caused and that may lead to line frequency pump apart from high effective scope. That is to say, there is no energy saving. In above case, we can take following measures to improve energy saving.

(1)    For small delivery water system, only a variable frequency pump is needed. High effective scope of pump should include state points which presents in high probability as many as possible.

(2)    Several pumps run in variable speed. Because of expensive price of VFD, user should consider economy comprehensively.

(3)    For line frequency pump selection, make system work in most disadvantageous point. The state point of pump should be close to left side of high effective scope as much as possible. If appearance probability of the most disadvantageous state is small, it can leave high effective scope and drop left outside of the scope. In this way, when system lift lowers, line frequency pump can still work in high effective scope.

2.2 Constant pressure variable flow system

Pressure transmitter is at outlet of pump for purpose of keeping outlet pressure constant. Generally, it is set as outlet pressure in the most disadvantageous state. Take example of mode: a variable frequency pump and a line frequency pump work in parallel. When flow reduces, variable frequency pump decelerates to keep lift unchanging and reduce output flow. Because outlet pressure is unchanging, output flow of line frequency pump does not change. Pump still works in high effective scope, thereby energy is saved. Note that when flow demand is small, outlet pressure is more than pressure demand. This causes static lift waste to some extent. There are two measures to improve:

a.       Enlarge diameter of pipe and make pipe curve smooth. This method will increase cost. User should make comparison with energy saving comprehensively.

b.      When user choose line frequency pump, make state point of each pump drop in high effective scope farthest.

3.      Explore the best mode of energy saving

We make contrast test for two modes.

-----A VFD runs in close loop, cooperating with other pumps of constant speed;

-----Two VFDs run in close loop and open loop respectively, cooperating with other pumps of constant speed.

We make several tests with same delivery flow and same pipe. Following is the statistic:

We get following from the table:

Two VFDs work: 155.9－146.9＝9 kWh/dam

One VFD work: 155.9－151.6＝4.3 kWh/dam

Therefore, mode of a VFD in open loop and the other in close loop is better.

4.      VFD advantages

4.1  Safety

Plant safety involves power safety and pipe safety. Technology of variable frequency can reduce impact on motor and power grid effectively and ensure smooth pressure, reducing probability of pipe cracking.

Start-up of big power motor affects power grid of plant. Generally, start current of constant speed pump is 6~7 times of operation current. The big impact causes trip-off easily and threatens power grid. Especially in the case of starting pump when grid current is close to full load. Total current is not over full load requirement, but big start current makes instant current beyond requirement. Pump can not be started. So production is affected. With medium voltage VFD, motor frequency rises from zero to operation frequency and current rises gradually. Impact on grid is eliminated. Power grid safety is ensured.

Pipe pressure is also an important index. High pressure causes pipe cracking easily and increases power consumption. Low pressure can not ensure water demand. Pressure requirement is different in different time. Before, all pumps are constant speed. As long as pumps start, they work in direct on line. The frequency and current are fixed. Mode of constant speed can not ensure pipe pressure in required scope. When flow demand changes, pressure changes. So work sequence has to be adjusted. That is not safe as well as economy. Furthermore, in instant moment of sequence adjustment, great changing of pressure leads to pipe cracking easily. Variable frequency pump can keep pressure constant. Adjusting frequency of pump can satisfy flow demand when pipe demand changes, so probability of pipe cracking is reduced a lot.

4.2 Energy saving and emission reduction

For variable frequency pump, energy saving is realized by reducing mechanical energy of unnecessary flow in pipe. Take an example, keeping outlet pressure at 0.40Mpa can satisfy flow demand. If the pressure is higher, flow increases little. That indicates water supply area is saturated. Pressure more than 0.40Mpa is waste. Power for maintaining this pressure can be saved by variable frequency motor.

Energy saving calculation:

Daily water supply: 250.6dam/day

Energy saved: 155.9－146.9＝9 kWh/dam

Daily cost saved: 250.6 dam×9kmh/ dam×0.523（average price）＝1179.57yuan

VFD works eleven months in a year, yearly cost saved:

1179.57×335 days＝395157yuan(USD 58110)

5. Concluding remarks

Water plant should select operation mode of pump reasonably according to pipe characteristics in order to reach purpose of energy saving in precondition of satisfying requirement.

With VFD, soft start avoids impact on motor insulation and reduces maintaince, saves maintaince cost, prolongs motor lifetime, as well as lowers wear and tear of pipe. It adjusts pipe pressure smoothly, saves cost 395157yuan (USD58110). In Shiyuan Water Plant, power cost accounts for 73.90% of production cost. Power saving can reduce production cost effectively, relieves inadequate power, makes pump station an saving pump station.