Horizontal multistage centrifugal pumps

When you hear “horizontal multi-stage centrifugal pumps?”, many people immediately imagine just a series of successive stages in a housing - like, the more of them, the higher the pressure, and that’s it. But in practice, especially when it comes to reliable operation under constant load, say, in water supply systems or on production lines, everything comes down to details, which are often in fine print in catalogs. The most common mistake is the underestimation of axial forces and balancing issues of the rotor assembly. You can assemble a pump with individually ideal wheels, but if you don’t calculate and compensate for the axial force from all these stages... it won’t work for long, the thrust bearing will quickly say “goodbye”.

From drawing to metal: where does the “devil” lie?

At our production site inZhejiang Yangzijiang Pumping Industry Co.,Ltdthe assembly line for multi-stage horizontals is a different story. It is not enough to simply have eight complete lines from R&D to service. It is important how the stages are combined. For example, after turning a shaft for steps, it is intermediately balanced - even before installing the wheels. Because if the shaft itself has even a minimal imbalance, then, after installing all the wheels and final balancing, such a vibration pattern may emerge that it is easier to reassemble. We went through this about ten years ago, when we were chasing the speed of assembly and missed this stage. The result is returns under warranty due to increased vibration after six months of operation.

Tests are a separate matter. Everyone at our standhorizontal multistage centrifugal pumpThey drive not only at nominal, but also at partial opening of the valve and at maximum pressure. You look at the Q-H graphs, and sometimes you see a failure or instability of the characteristic precisely in the low-flow zone. This often indicates problems with the NPSH in the first stage or the inlet geometry. We have to return to the designers and make adjustments to the body casting gating system. This is not a quick process, but otherwise the pump will make noise and be destroyed from the inside, and the client will be dissatisfied.

Diffuser and impeller materials are another key consideration. For clean water, cast iron can be used, but we often have orders for pumping process fluids, say, with moderate abrasiveness or temperature. Here we are already looking at stainless steel grade 304 or 316, and sometimes at special alloys. It is important that the material of all stages in the pump is the same in its mechanical properties, otherwise it may jam due to thermal expansion. Once there was a story with a pump for circulating water, where the third stage (spare) turned out to be from a slightly different batch of stainless steel - the expansion coefficient was different. During thermal shock, the rotor touched the housing. I had to change all the wheel assemblies.

Assembly and pre-launch checks: something that is often missed

There is an unspoken rule in the assembly shop: before installing each impeller on the shaft, check the seats for burrs. Seems like a small thing? But one small chip remaining after processing can create a micro-distortion of several tenths of a millimeter. And when there are eight or ten such stages, this misalignment is multiplied, and the rotor becomes a “screw”. We set the axial clearance with feeler gauges, but not blindly according to the passport data, but adjusted for the thermal expansion of the specific material of the shaft and wheels that went into this assembly. When cold, we always make the gap a little larger.

Seals. This is a sore subject for horizontal multistage pumps. Stuffing box seals are cheaper but require maintenance and leakage. Mechanical mechanical seals (TMU) are more reliable, but are critical to the cleanliness of the pumped medium. We are inYangzijiang Pumping IndustryWe often accommodate the client halfway and offer a configuration with double TMU and barrier liquid, especially for critical applications. But it is important to convey to the customer that this unit also needs to be monitored - the pressure in the barrier cavity, the cleanliness of the liquid. Otherwise, the expensive seal will fail in a matter of hours, and the “unreliable pump” will be to blame.

Before shipment we always do a test run on the water. But not the usual one, but one whose temperature is close to what is expected during operation. This is important for checking thermal clearances. It happens that the pump is assembled perfectly “when cold,” but after reaching operating temperature it begins to knock or vibrate. Such a copy is sent for revision, not to the client. It's better to lose a day on sorting than a reputation on a return.

In the field: installation and first problems

The most common mistake on site is misalignment with the electric motor. For a single stage pump, a slight misalignment will probably pass. But for a long rotor of a multistage horizontal pump, this is fatal. The vibration will be such that it will destroy the bearings and seals very quickly. We always insist that installers perform adjustments not only in a cold state, but also after warming up the system, at operating temperatures. The metal expands, the foundation slab may “sag” a little.

Another point is the pipe loads on the connecting pipes. The pump is not a support for the pipeline! If the pump flanges are subject to bending moments from the weight or temperature deformations of the pipes, this is transmitted to the housing and can cause it to misalign relative to the rotor. We saw a case where a pump, which worked for three years without complaints at one facility, after reconstruction of the piping, failed within a month. The reason is that new pipe supports were not installed near the pump, and the entire load fell on its body.

Start. You should never suddenly open the outlet valve of a horizontal multistage centrifugal pump, especially if it is working against a closed valve (in a closed circuit). The pressure increases instantly, and there is a risk that the rotor, with its axial clearance, will experience a shock load. We always recommend soft starting, preferably with a frequency converter. Yes, it's more expensive, but for equipment that needs to last for years, it's a worthwhile investment. On our websiteyangtzeriverpump.ruIn the section with technical recommendations, we try to describe these nuances in detail, but, alas, not all site engineers read them.

Service and feedback: what they bring from the fields

In the experience of our service department, the main problems with returnable pumps are the consequences of improper operation. Abrasive wear of the flow part if a filter has not been installed. Cavitation destruction of the first stages due to insufficient suction pressure. But there are also constructive lessons. For example, several years ago we received a batch of complaints about increased noise in a certain range of operating points. Several pumps were disassembled and there was no visible wear. After lengthy tests, it turned out that at a certain combination of flow and pressure, hydrodynamic instability arose in the diffuser channels of one of the intermediate stages. The geometry of the diffuser was slightly adjusted in the new revision of the casting equipment - the problem went away.

This experience is now being incorporated into new projects. Our R&D engineers don't just draw hydraulics, they regularly communicate with assemblers and service engineers. Because sometimes simply replacing a standard bearing with a more heat-resistant one or changing the method of fixing the unloading disk (axial force unloader) increases the service life of the entire unit several times. And you won’t find this in textbooks, this is knowledge paid for with breakdowns and complaints.

Now, looking at our lineuphorizontal multistage centrifugal pumps, I understand that each standard size is not just numbers in a catalog. This is a story of improvements, bugs fixed, and trade-offs made between cost, reliability, and efficiency. We, as a manufacturer, are responsible not only for ensuring that the pump pumps water, but also for ensuring that it does so quietly, for a long time and without surprises for the person who bought it and installed it in their system. And this is perhaps the most difficult thing in our work - to predict all the conditions in which this piece of iron will find itself, and make it ready for them.

Instead of a conclusion: thinking out loud

Sometimes I look at the finished pump before shipment - massive, painted, with a tag. It seems like just a product. But inside it are dozens of precisely fitted parts, strength calculations, and bench tests. And the most important thing is the experience that goes into it. Experience of past failures and successful decisions. Therefore, when a client asks why the same model differs in price from different manufacturers, I would like to talk not about the cost of cast iron, but about all these stages: intermediate balancing, thermal running-in, geometry correction based on field failures. This is the very added value that distinguishes a simple product from a reliable tool. And this, perhaps, is our task inZhejiang Yangzijiang Pumping Industry Co.,Ltd— to make not just pumps, but devices that, after installation, simply work, allowing people to forget that somewhere there is our horizontal multistage centrifugal pump and is doing its job properly. I think this is what we should strive for.