See the necessary care in installing the motor pump to avoid these problems that you may have never even heard of.
(Vilmar Rossi – Engineering)
If you follow our content, either here on the blog or on our social networks, you should already know what needs to be taken into account when choosing the right pump model to meet your needs and the requirements of the hydraulic pumping system with which you're coping, aren't you?
Starting from this point, that is, having defined the right product, it's time to start installing and to start this very important step, first of all it is necessary to analyze factors such as:
- The installation site, to see if you need to prepare the ground or any compartment to protect the equipment;
- The existing and required electrical installation;
- The existing and required hydraulic installation;
- The technical standards to be followed when installing the motor pump, based on ABNT and Ministry of Labor guidelines; It is
- Technical issues that will ensure proper functioning.
In order not to take unnecessary risks or compromise the product warranty, it is essential that the instructions given by the equipment manufacturer are always followed with regard to the location and conditions of electrical and hydraulic installation, as well as the start-up. To ensure that all these points are considered, reading the product manual is essential, so DON'T SKIP this step, agreed?!
With regard to technical issues to ensure the proper functioning of the pump, we understand that it is important to avoid two of the main problems that, if not taken care of, can interfere with the operation and even the inoperability of the equipment. See what they are below.
cavitation
Cavitation is the phenomenon of vaporization of a liquid by pressure reduction during its movement. In a pumping system, the vaporization process will start when the liquid in the suction section (water inlet) reaches the vapor pressure for that temperature; part of this liquid will vaporize, forming small bubbles or “cavities” inside which the liquid will vaporize. The vapor bubbles are conducted by the liquid current caused by the movement of the rotor or by the fluid impulsion itself, until they reach pressures higher than the vapor pressure, which leads to the implosion of the bubbles and consequent condensation of the vapor and return to the state liquid.
Cavitation is a relatively “normal” phenomenon that occurs in the region of the pump impeller. However, it can become problematic when, at the time of the change of physical state (from steam to water), the fluid, already at high speed inside the rotor, causes pressure waves of such intensity that they overcome the resistance to traction of the rotor material, being able to pull particles from the body, blades and walls of the pump.
The effects of cavitation depend on the time of its duration, its intensity, the properties of the liquid and the resistance of the material. Erosion is a very common effect of cavitation and occurs where the bubbles implode rather than where they form. In addition to erosion, cavitation can increase the corrosion process, wear and remove particles and pieces from the impeller, seals and casing, impacting the drop in pump performance, vibrations and noise.
At this point you may be wondering how to identify if a pump is cavitating. The simplest way to perceive this phenomenon is by the sound it will make. That's right, for the noise! The noise of a cavitating pump is different from the noise of its normal operation; when it is cavitating you hear a sound that will give the impression of pumping sand, gravel or other material that causes impact. That sound is actually the air bubbles imploding inside the rotor!
Identified the problem, what to do to solve it?
To avoid cavitation, it is necessary to reduce the suction force and this can be done in different ways, see some alternatives:
- Bring the pump closer to the liquid;
- Increase the pipe diameter or eliminate some connections;
- Reduce the flow, decreasing the actuator rotation or closing the valve in the discharge;
- Reduce vapor pressure by lowering the temperature of the liquid; or
- Increase the pressure on the surface of the liquid in the suction reservoir, when possible, in the case of a closed reservoir.
Another way to eliminate the motor pump cavitation problem is to change the diameter(s) of the pump impeller(s). But attention: this alternative should only be adopted as a last resort, as this change can substantially alter the hydraulic performance of the set.
To avoid headaches. the best thing is to find out before installation whether a pump will cavitate. To do this, it is necessary to perform a calculation to evaluate the conditions of vaporization of the liquid. This is the NPSH (Net Positive Suction Head), term in English without direct translation into Portuguese. It will be necessary to compare the available NPSH and the required NPSH. Don’t worry, let’s explain what these two are:
- The available NPSH (NPSHD) indicates the absolute pressure exerted at the pump inlet and must be calculated using a formula that we will introduce you to.
- The required NPSH (NPSHR) is the result of the pump design and indicates the minimum value for it to reach the expected hydraulic performance without cavitation occurring. It is the pump manufacturer who needs to inform this value in the technical material of the product.
In theory, to ensure that the pump works free of cavitation, it is necessary that the available NPSH is equal to or greater than the required NPSH. In practice, it is recommended to consider a safety margin of at least half a meter of water column (0,5 mca) beyond the NPSH value required to avoid cavitation.
To determine the available NPSH value, the conditions of local atmospheric pressure, vapor pressure of the pumped liquid and the total head of suction must be considered, as follows:
NPSHD = H0 – HV – PCS ± AS
being:
H0 local atmospheric pressure in relation to altitude;
HV the vapor pressure of water in relation to its temperature;
PCS pressure loss in suction; It is
AS the suction height.
After calculating the available NPSH, if it Do not be greater than or equal to the required NPSH plus the safety margin (0,5 mca, as suggested) and it is not possible to change the installation of the system to avoid cavitation, check the possibility of using a pump that has a lower required NPSH .
Water hammer
Water hammer is a hydraulic phenomenon caused by sudden pressure changes in the pipes of a pumping system. It occurs as an effect of an obstruction of the fluid, due to the abrupt closing of a valve, due to the accumulation of air inside the fluid or even when the motor pump is turned on or off without an acceleration and deceleration control.
The velocity energy, transformed into pressure energy, causes the fluid, returning in the opposite direction to the flow, to collide with the fluid that is still trying to flow in its “normal” direction, creating shock waves along the pipe, which may cause deformation or even rupture of the walls of the hydraulic components.
Such a blow is often accompanied by a noise that gives the impression of something similar to a hammer hitting the inside of the pipe, hitting pipes, valves and connections. As a curiosity, ram is a kind of hammer, hence the name of the blow!
So that the pumping system is not damaged, it is important to prevent water hammer and for this there are some practical solutions:
- Use pipes with large diameters, thereby reducing the flow rate;
- Use check valves every 30 meters of the discharge pipe; or
- Install a frequency inverter or a soft starter in the electrical panels that drive the pumps.
Most of the problems seen in pumps are due to inadequate model selection as well as incorrect installation and maintenance of the pumping system. In applications that pump clean water at ambient temperature, the service life of a motor pump is basically determined by the life of the bearings. In other applications, the wear of mechanical seals and engine problems are the most commonly found failures, but this is already a subject for another post!
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