a comparison of pumps seal in chemical plant

7/28/2019 A Comparison of Pumps Seal in Chemical Plant

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A comparison of pumpswith different sealing systems typically used in a chemicals plant

By Christian Huber, Head of Preventive Maintenance, Wacker Chemie GmbH, Burghausen

Strategic maintenance ensures corporate successIn a chemical plant, centrifugal pumps with drive and seal concepts based on the mechanical seal,magnetic coupling and canned motor were compared in terms of maintenance. The resultsindicated that the maintenance costs for a mechanical seal are comparable or lower. Basis of thecomparison were the recorded total maintenance costs for pumps which were in active operationthroughout the study period (interrupted only for repair purposes). For better comparison, an indexwas introduced based on the repair costs per annum and drive rating for pumps of a specific type.

Maintenance strategies and costs have a major impact on corporate success. Chemical plantshave a particularly high quota of pumps in their actual production facilities as well as in their peripheral infrastructure. Keeping the outlay for maintenance and availability as low as possible is acentral concern of technical departments and, particularly as regards availability, exerts a directeffect on plant productivity. Special attention is paid, therefore, to the correct choice of pump.

Particularly in those areas where standard chemical pumps are often used, technologicaldevelopments over the past few years have resulted in severe competition between designs withsealing systems based on the "mechanical seal", "canned motor" and "magnetic coupling" for comparable applications. Pump manufacturers, owner-operators and research institutes have allconducted series of comparative studies on the subject of "optimal pump selection" using the mostdiverse criteria, the most popular of late being the life cycle analysis. We would also like tocontribute to the discussion of drive/seal concepts in chemical pumps with the followingcomparison of recorded maintenance costs.

Comparison of various sealing systemsThe idea behind the comparison was to record all the maintenance activities and analyze the

pumps actually used in the study period rather than evaluate different types of pump seal in thelight of typical cases of repair and failure statistics. The findings should then be applied to the totalmodel complement and be standardized for nominal ratings.

Of our recorded complement of currently 18,000 pumps at the Burghausen works, a total of 7,250pumps were in active operation in production and supply plants during the study period of 2.5 years(installed in technical positions in the SAP-R3 data system; sometimes interrupted for repairs). Of these 7,250 active pumps, 4,185 (58%) belong to the category "centrifugal pump". A further 25types of pump are differentiated by catalog list. In the evaluations it should be noted that the dataexisting for some pump types is minimal and therefore not representative.

To obtain a broad data base, all the maintenance costs incurred over a period of two and a half years for the above complement of pumps were evaluated. The costs cover not only the frequently

recorded pure costs of repair in the pump workshops but all the outlay recorded in connection withthe repairs, e.g. the work performed by electricians and measurement and control technicians (e.g.disconnecting canned motor pumps), transport operations, mechanics, etc.

The ideal basis for drawing comparisons would exist if different drive/seal concepts were used onidentical pump stands under identical conditions. In reality this is rarely the case and so cannot beevaluated. We have resorted, therefore, to comparing identical types of pump (e.g. standardchemical pumps) with different drive/seal concepts.

In this comparison it is necessary to allow for the following factors:

Canned motor pumps and pumps with a magnetic coupling represent just a fraction of the

pump applications with a mechanical seal because the applications are limited to cleanmedia and – in the case of the magnetic coupling – to the absence of ferrite contaminants.



Compared to the mechanical seal, canned motor pumps are also restricted in use wherecertain aggressive, caking, sticking and very hot media or ultrapure media are involved.

A major, immeasurable proportion of pump damage incidents is owed to faulty operation.Canned motor pumps and pumps with magnetic couplings are highly protected by processcontrol measures from dry running and from running outside the permissible characteristicrange.

The output range of a mechanical seal is far bigger. This is taken into account in thediagrams by plotting the costs as a function of output. It is assumed that repair costs arelinearly dependent on nominal pump output.

In the following diagrams the seven items concerning the standard chemical pump, the centrifugalpump and the standard water pump are of interest. Other types of pump are either too special for comparison in the drive/seal concept or are underrepresented in the data base or are not availablein an alternative version to the mechanical seal.

Of the complement of centrifugal pumps covered by the study, 49% are equipped with amechanical seal, 43% with a magnetic coupling and 7% with a canned motor. Limiting the study tostandard chemical pumps results in a breakdown of 27% mechanical seals, 65% magneticcouplings and 8% canned motor. On the whole, therefore, the mechanical seal is on par with theother sealing concepts, whereas for purely chemical-specific applications and lower outputs itsshare is smaller. Other weightings are known to us from other member plants of the VCI(Association of the German Chemical Industry). These weightings are closely related to theproduction spectrum and to the pump purchasing policy.

Repair frequency must be considered according to pump application. In the case of "standardchemical pumps" the magnetic coupling and the canned motor have a repair frequency of 14%, far better than the mechanical seal with 25%. This could be partly owed to the cleaner media andbetter protective measures than those in mechanical seal applications. If mechanical seals areoperated with better suited media, as is the case with standard water pumps, then the repair ratestend to correlate again. There is insufficient data for comparing the repair rates of magnetcouplings and canned motors when used with water.

Repair costs of the various sealing systemsThe costs of pump repair, obtained by adding up all the recorded costs incurred, are remarkablyhigh compared to the value when bought. Breakdown of the costs into central workshop,replacement parts and miscellaneous logistic functions will not be examined at this point. In thecomparison of seal concepts, repairs of pumps with mechanical seals are cheaper (by a third for standard chemical pumps) mainly on account of the lower cost of replacement parts. If this is put inrelation to the frequency of repairs per year, the annual type-related maintenance costs of the threeseal concepts for standard chemical pumps tend to correlate again on account of the higher maintenance rates for mechanical seals. Standardizing the maintenance costs as a function of nominal output now results in the annual complement-related maintenance costs per type and KWoutput. The higher average nominal output of pumps with a mechanical seal in the standardchemical sector means that the comparative figures then work out at 92 DM/year + KW for

mechanical seals, 104 DM for magnetic couplings and 67 DM for canned motors. By comparison,the characteristic value for a standard water pump with a single mechanical seal works out at just15 DM.

Further aspectsFor the now customary analysis of life cycle costs it is necessary to add the servicing costs, initialpurchasing costs and energy overheads to the repair costs. The initial purchasing costs are lowestfor pumps with a mechanical seal, but correlate again if a multiple mechanical seal with buffer system is required. Notable servicing costs are incurred in connection with mechanical seal buffer systems. In the case of pumps with a magnetic coupling it is recommended to inspect the state of the magnet driver's antifriction bearing at regular intervals using vibration measurements. Thisgreatly reduces the costs of consequential damage resulting from damage to the can due to adefective antifriction bearing. The energy costs for pumps with a canned motor or magneticcoupling are higher than for pumps with a mechanical seal on account of the larger gap and eddycurrent losses respectively. If the power loss of a single magnetic coupling equals 5%, for example,



then the extra energy costs within the repair-free intervals will tend to equal the repair costs (basedon the state of the art of the sometimes old pumps in the study).

ConclusionIn the "standard chemical pump" field of application mainly used for the purpose of this comparison,pumps with a mechanical seal are at a slight advantage in terms of operating costs when viewed

overall. When the above aspects are considered, there is a statistical cost advantage for themechanical seal. For the sub-field where the used of multiple mechanical seals with a buffer system is prescribed for safety reasons, it usually seems to make more sense with a view to costsand other aspects for users to install canned motor pumps and magnetically coupled drives,particularly where small pumps are concerned.

a comparison of pumps seal in chemical plant

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