Research and production Centre ANOD
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PRODUCTION

Pump modernization

Vladimir Makoldin, Head of the Design Department of Dynamic Equipment

Competitiveness of an enterprise directly depends on reliability of technological and pump equipment. That’s why pump units require excelled durability and safety characteristics. Mechanical engineers were issued a challenge to increase pump overhaul life up to 5 years. Rich experience of ANOD, while dealing at the mechanical seal market, demonstrates that the weakest points of present centrifugal pumps are assembly seal systems. Judging by operation results of some pump units, it was established that one of the main reasons for mechanical seal and bearing assembly failures is an exceeded flow rate, which is not specified for this pump size. While operating at low flow rates, medium recirculation is performed in a pump pressure casing (the so-called backflow). The backflow stimulates high hydrodynamic vibration level.

Besides, at low capacities radial and axial forces upon a pump rotor are maximally increased. That results in shaft deformation and bearing assembly overload and subsequent wear-out. For a mechanical seal, being installed onto a deformed shaft, it shall not operate properly in conditions of vibration, high temperature and pressure (see Fig.1). Rotor and pressure casing defects, wear-out of impeller’s groove seals, absence of rotor’s dynamic balancing – all this just worsen the problem.

So, the main reasons for assembly seal systems failure are operation at non-specified flow rates and increased vibration level.
Taking into account the actual examples of pump exploitation, ANOD performs pump modernization by means of a regular seal-bearing assembly change into the bearing-seal block BPU.

Pump modernization successfully solves the problem of seal-bearing systems where assembly’s service life, at the hardest service conditions – increased hydrodynamic vibration, is nearly equal to that one of a pump pressure casing.

As an example, let’s take modernized NKV 600/320 pump model (see Fig. 2). Opinions of mechanical engineers, regarding pump operation at non-specified parameters, are mostly negative. The overhaul life, at such parameters, is only several months. If modernized, the pump’s rolling bearing assembly is to be changed into BPU. Constituting one unit, BPU combines two single mechanical seals, contour and atmospheric ones, with radial and axial plain bearings installed between them. Lubrication and cooling of BPU are performed by means of a clean buffer fluid. With BPU installed, NKV 600/320 has been operating without any stops since 2007.

In comparison with rolling bearing, plain bearings possess higher support ability. Besides, they are able to dampen rotor’s multivibration fluctuations and provide for better vibration characteristics. As a result, an overhung pump unit with BPU has a reduced vibration level.
Mechanical seals in BPU unit is located next to a radial plain bearing. That is why shaft deformation, at the point of bearing’s installation, is almost completely excluded.

As friction pairs of plain bearings by R&PC “ANOD” the most wear-resistant materials are used: silicon carbide - silicon carbide. Such bearings are capable of working in any liquid, including water, at the specific loads of 50…100 N/cm2, and not being subject to wear-out. Plain bearings are not susceptible to abrasives of the liquid they work with.

It’s worth mentioning that while designing of silicon carbide plain bearings for high temperatures, the following physical properties of materials are considered:

  • difference of thermal expansion coefficients of silicon carbide and steel;
  • silicon carbide’s good capability of compression, but not of tension or bending.

R&PC “ANOD” developed silicon carbide radial bearings (see Fig. 3) that are to operate at environment temperature of 200°C. When working at such high temperatures, the ceramic elements are subject only to compressive loads, if standard radial clearance in a friction pair is preserved the same at any operating mode.

In comparison with rolling bearing, plain bearings possess higher support ability. Besides, they are able to dampen rotor’s multivibration fluctuations and provide for better vibration characteristics. As a result, an overhung pump unit with BPU has a reduced vibration level.
Mechanical seals in BPU unit is located next to a radial plain bearing. That is why shaft deformation, at the point of bearing’s installation, is almost completely excluded.

As friction pairs of plain bearings by R&PC “ANOD” the most wear-resistant materials are used: silicon carbide - silicon carbide. Such bearings are capable of working in any liquid, including water, at the specific loads of 50…100 N/cm2, and not being subject to wear-out. Plain bearings are not susceptible to abrasives of the liquid they work with.

It’s worth mentioning that while designing of silicon carbide plain bearings for high temperatures, the following physical properties of materials are considered:

  • difference of thermal expansion coefficients of silicon carbide and steel;
  • silicon carbide’s good capability of compression, but not of tension or bending.

R&PC “ANOD” developed silicon carbide radial bearings (see Fig. 3) that are to operate at environment temperature of 200°C. When working at such high temperatures, the ceramic elements are subject only to compressive loads, if standard radial clearance in a friction pair is preserved the same at any operating mode.

Results of modernization:

  • higher shaft rigidity due to maximally close installation of a radial bearing to the impeller and increased distance between bearings;
  • no oil lubrication of bearings;
  • cooling and lubrication of interacting surfaces of bearings and mechanical seals is performed by means of a clean buffer fluid of the support system;
  • increased service and overhaul life of pumps.

A certain range of BPU units was developed to be installed into a pump scope of Volgograd Neftehimmash plant (NK, NKV, TKA pump types) (see Fig. 4). Some BPU models are successfully operating together with pressure casings by Klaus Union HMD, Worthington, Moldovahydromash, Yuzhhydromash, etc.

The scope of ANOD’s activities also includes modernization of condensate, feed and booster vertical/ horizontal between-bearing multistage pumps (see Fig. 5, 6). The problems of such pumps are the same: short overhaul life because of bearings and mechanical seals failure.
In such pumps standard bearing assemblies cantilevered in the casing, are changed into the blocks. Every block unit consists of an axial single-acting bearing, radial bearing and mechanical seal.

Axial bearings fix the rotor, relatively to the casing, in axial direction and receive unbalanced rotor’s axial force. Due to diminished between bearing distance, shaft rigidity is 1,6-2 times as high. Friction pairs are lubricated and cooled by means of a pumped medium. The circulation is performed by means of impeller, installed into BPU, when a pump is an operating mode. If a stand-by mode, the circulation is made with the help of natural circulation. Mechanical seals are installed at the pump periphery, which makes the maintenance easier.

As a result, modernized between-bearing pumps do not require oil lubrication system; they have increased service life, improved vibration characteristics and reduced maintenance costs.

It’s worth mentioning that not only deteriorated pumps are being modernized by R&PC “ANOD”, but new ones as well, with pressure casings by Russian or foreign manufacturers.

So, modernized pump units satisfy contemporary technical requirements: high reliability, safety and durability.

R&PC “ANOD” remarks that pump designers and enterprises, which apply pump equipment, should take decisive measures of using the most contemporary technical innovations in different fields of industries.