AUTHOR: BRYAN ORCHARD
Against a background of rising energy prices and the greater pressures to reduce CO2 emissions, it is becoming increasingly important for all sectors of industry to consider the operating efficiency and energy consumption of pump systems. Many pump manufacturers are doing their utmost to drive down energy consumption costs by offering energy efficient motors, matching impeller diameters to actual operating conditions and recommending the use of variable speed drives. This approach is to be applauded, but if pump users do not create the best operating conditions and look after their pump systems the opportunities for reducing operating costs are compromised.
Pump systems are large consumers of electricity and their operation will always have an impact of the price of the final product or service. A pump that is matched to the application will contribute to energy savings because its performance is tailored to the needs of the system. Selecting an oversized pump in the belief that one day it will become usefull if production is increased or for safety reasons is a false economy. Similarly, running a pump without any checks on component wear and maintenance will reduce operating life and increase operating costs over the medium to long term.
It is inevitable that pumps will display signs of wear, as it is almost impossible to design a pump that is free from operational deterioration. Wear will have an adverse influence on the performance of the pump, causing mechanical losses, leakage and energy (hydraulic) losses. Components most likely to be affected are bearings, mechanical seals, wear rings, rotating elements and the shaft. Then there is the matter of the internal surfaces which can be subjected to corrosion and erosion by the pumped fluid (Fig. 1). Surface conditions will have the greatest impact on energy losses.
The issue of equipment operating lifetime and downtime costs are issues that are receiving long-overdue attention within all areas of processing and manufacturing. Whereas at one time only the initial investment cost was the main factor to be considered in selecting pumps, lifetime costs are now very much a part of the specification process. It is a fact of life that deferring pump maintenance is often used as a means of reducing costs, but this can be a false economy. Not only can the life of the pump be reduced, worn or damaged parts will affect operating efficiency and this will inevitably impact on other operations in the production/processing cycle. Pump failure, due to lack of regular inspection and maintenance, can lead to plant shutdown and excessive down-time costs.
Many pump manufacturers and suppliers believe that the best way of ensuring trouble free and low maintenance pump operations is to invest in careful pump selection and proper installation at the outset and then to monitor changes in performance at regular intervals. If the pump has been sized for the application, dynamically balanced, installed on robust foundations in order to avoid vibration and operated in the correct manner appropriate to the application then a major failure should never be experienced. This may add costs to the initial investment, but over the long term the investment will be paid back through energy efficiency, low downtime and reduced maintenance costs. Unfortunately not all pumps are operated to their best efficiency point or they are left to operate unattended for long periods, so when the pump does eventually crash it is either replaced or fitted with replacement parts without any investigation as to the causes of the failure.
To plan or not to plan – that is the question?
There is of course an argument for restricting planned and preventative maintenance. In other words; ‘if it ain’t broke, don’t fix it’. Too much maintenance intervention can result in damage to components when they are removed and reinstalled. This is particularly apt where the mechanical seal is removed. Misalignment when the mechanical seal is being reinstalled will cause excessive movement and eventual failure of the seal’s faces.
Striking a balance between preventative and condition-based maintenance lies in careful monitoring of the pump performance, so that if any symptoms of a problem are noted action can be taken that prevents total pump failure. The pump should only be taken out of service for maintenance after a detailed study of the symptoms and causes has been carried out. If there has been physical or mechanical damage of the pump’s components, then a review of the process operation needs to be undertaken in order to identify the cause of the damage.
Preventive, predictive, and prevention/pro-active maintenance practices generally require the pump operator to have an in-house team of qualified employees or to have maintenance contracts with the pump manufacturer. However, this is not always feasible and that is where outsourcing this work to a knowledgeable contractor who can work closely with the maintenance personnel on determining failure and then assist in the repairs or design changes if these are required.
AxFlow managing director Tony Peters believes that when a service organisation is called in to provide both on-site or off-site maintenance and repair, it should be standard procedure to examine the pump in detail and to discuss with the customer the application and operating conditions. In this way the service provider is able to give a wholly effective solution to the problem. Where feasible, his service team undertakes remedial work at the customer’s premises and typically this will involve identifying the cause of the problem if there has been a pump failure.
When called in for a pump refurbishment project the first challenge is not knowing what the condition of the internal components will be until the complete unit is stripped down. Externally the pumps may look to be either in a reasonable state or not worthy of being rescued. A thorough examination of the pump in its operating environment will reveal the cause of the failure and establish whether or not the pump can be repaired in-situ. Engineers should have a check list of actions that are adhered to during the disassembly process and it is this procedure that is crucial to identifying where problems lie. Taking photographs at each stage of the stripping down procedure allows component appraisal. Using this visual data is deemed essential when it come to making recommendations as to the best course of action to be taken.
According to Mark Redgrove, AxFlow’s’ technical support manager, the goal for the service company must be to get the customer back up and running as quickly as possible, but not at the expense of creating further problems that might arise in the future. Pump replacement may at first seem the easiest option, but this can be costly for the customer and may not always be the best long term solution.
Case study panel
During 2010, AxFlow was involved in the refurbishment of the four Gwynnes 24-inch and 48-inch mixed-flow vertical land drainage pumps operating at the East Hull pumping station since 1949. An external inspection indicated that the pumps were old and in need of replacement and had exceeded their design life capacity. Also, they were no longer being manufactured.
In late 2009 an on-site evaluation of all four pumps in the East Hull works enabled recommendations to be made as to what work would need to be undertaken to put them all back into a reliable and efficient operating condition. When later removed from service for a thorough evaluation it was clear that they could be renovated to a standard that would meet the future requirements of the Environment Agency. Most importantly, the service company was able to undertake the renovation project in a time frame quicker than a full replacement programme and at a more competitive price.
In mid-January 2010, with assistance from AMCO, the the four pumps (Fig.1) were removed and brought to AxFlow’s Huddersfield Service Base where a full strip down was undertaken to allow dimensional checks to be made along with an overall condition report. The two 24-inch pumps were found to be in a reasonable condition, but the bronze bushes, sleeves and grease-lubricated bearings were fairly well-worn but were serviceable. The two 48-inch pumps did need more work and it was necessary to cast a new wear ring for the back of the impeller on the third pump as the original was cracked. When the impeller race was removed on this pump it completely disintegrated.
The service team balanced the impellers and replaced all of the wearing parts such as bushes and sleeves on the shafts (Fig.2) on all four pumps. In addition, all the white metal Mitchell bearings were overhauled, the pads re-metalled, thrust collars skimmed and all the internal surfaces of the housing shot-blast and repainted.
The key to the success of this contract was being to able meet the Environment Agency’s request for work on all four pumps carried out simultaneously, as opposed to instigating a rolling programme. Because of the size of the pumps, weighing up to 12 tons, it was necessary to dismantle them on site then rebuild them back in Huddersfield once the refurbishment jobs had been undertaken to check tolerance and dimensions (Fig. 3). The pumps had to be dismantled again for return to the pumping station and once again be rebuilt on site. From first day onsite through to having the pumps re-installed and ready to run took just 12-14 weeks
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