In his pumping systems column this month, Harry Rosen’s tells a fishy story and relates it to the age-old practice of throttling discharge valves to limit the current drawn by pump motors.
A friend of mine hails from the West Coast and recounted this story to me a while ago. Being close to the sea, fresh fish was always a big part of his menu and, for years, he observed his wife trimming 2-inches of prime flesh off the tail while preparing the fish to put in the oven. One day he thought to ask why the strange preparation ritual? He was told that this was the way her mother had taught her, who had been taught by her mother.
It was a tradition in their family going back to her great grandmother and nobody had ever thought to question it. On further investigation, her great grandfather had been a fisherman and would always return home from a trip with a large fresh fish. Unfortunately the largest dish they had was 16-inches long so she had to trim the tail to fit it into the oven. This physical limitation had evolved over time into a strict requirement ingrained into the families cooking.
This got me thinking of a practice I see in many of the plants I visit – throttling the pump to limit the amps drawn by the motor. As in the case of the fish story above, the original reason for throttling the pump has often disappeared in the midst of time. Peering through our looking glass, perhaps some of the following reasons could have applied:
- Once upon a time throttling was required to achieve a certain duty and it was convenient to use amps as a visible indicator to determine how much to close the valve. When the duty requirement changes, though, the unwritten rule to keep the amps below a certain value remains.
- Someone thought it was the maximum rated amperage of the motor.
- A bright spark thought he could increase motor efficiency and extend life by running at 90% of rated load – which is 100% wrong!
- Perhaps a rewound motor was uses at some stage.
- The pump was considerably worn and drawing more amps than normal. Since then the pump has been refurbished.
- One year they had a really hot summer and a lack of ventilation within the pump house led to insufficient cooling and the motors tripping.
- And finally, the genuine case: running the pump in its unthrottled condition draws more current than the motor can handle.
What is the problem with throttling anyway? Anyone who has attended a course on pumping system efficiency will know that the specific energy of a system increases when valves are throttled. This is because the additional friction loss through the valve adds additional head to the pump duty, thus consuming more power in order to pump the same amount of fluid. This is clearly shown in Figure 1, where the amount of useful energy – the energy being used to provide actual pumping work – is only a small proportion of the total energy being consumed by the pump.
In cases where the pump is being throttled to reduce the flow rate to a downstream process, pumps have to be oversized to deliver more flow than the system requires. Throttling the pump, however, is often seen as a practical, though inefficient, solution. When the system demands a higher flow rate than the pump is currently delivering, as is often the case in cooling water systems, if the pump is still being throttled then this is just throwing energy and money away.
A case study: throttled pumps at a steel mill
Steel mills produce a huge amount of heat and cooling water systems are often some of the biggest consumers of power in the plant. One such system investigated was circulate water using eight pumps in parallel, of which six were feeding cold water through the shells of a continuous arc furnace. During a plant walkabout, we noted that three of the six pumps were being throttled by 50%, while one valve was almost 75% closed. According to the operators the reason for the throttling was to keep the amps drawn below a certain value, but they did not know what value this was and the current could only be measured in the motor control centre. So they ‘just knew’ to keep the valves in the throttled positions noted.
The maximum rated current for the 3.3 kV pump motors read off the nameplate was 60.5 A, but when we measured the actual current drawn in the MCC (motor control centre), all reading were around 56 A.
We sequentially opened each of the valves and checked that the amps drawn were less than the maximum motor ratings. Unthrottled, all the motors were well within their ratings. In addition, when all the valves were 100% open, we were able to achieve the required flow rate with one pump shut down completely.
Running five pumps instead of six led to savings of 258 kW, which translates into 2.1 GWh in reduced energy costs per year – and this return from a zero cost intervention.
Why were the pumps being throttled? There may have been a reason at some point, but things change and if the consequences are not being measured, how can pump operators know that throttling every pump still has any beneficial effect? Even if there was a valid reason for throttling at some point, shouldn’t it have been investigated to determine whether that reason still applied? Or do we continue to throw away two-inches of prime fish because this is what we have always done?
But what happens in cases where the motor is drawing more amps than its allowable maximum. Surely this is a case when throttling a pump can be justified?
Wrong! There are other ways to fix such situations and none of them involve wasting energy through throttling. This is a topic for the next article, however, when we will discuss energy efficient ways to reduce pump output, such as trimming the impeller, reducing the pump speed or changing the pump.