avitation in pumps
In a hydraulic system, as the fluid velocity increases, the regional pressure decreases to the extent that it is equal to the fluid pressure in that region. It evaporates (called vapor pressure), if evaporation occurs inside the pump, ie the bubbles reach a higher pressure area after a short path and explode, creating shock waves that strike between the fluid boundary and the structure. Causes erosion and corrosion.
Since the contact surfaces of such bubbles with the pump blades are very small and a very large force is applied to the blades due to this explosion, it causes the blades to corrode and gradually larger cavities are formed. This stage is called Cavitation Pitting or Cavitation Erosion. مینامند.
The following figure shows the collapse of a cavitation bubble near a solid boundary.
Factors affecting the cavitation phenomenon:
1- Geometric factors include:
1-1- Surface roughness of the blades, especially localized bumps and depressions
1-2 Construction and crystal joints
1-3-Deflector & flow splitter
1-4-Pipe & Port of Ducts
1-5-Input nozzle shape
1-6-Change of water passage shape (Change of water passage shape)
1-7- Misalignment of conduit in the waterway (Misalignment of conduit)
2- Hydrodynamic factors:
2-2-Fluid flow rate
2-4-Border layer development
3. Other factors:
3-1-Heat transfer during pumping operation
3-3-Number and size of bubbles in the fluid
The types of cavitation that may occur in pumps are:
Evaporative cavitation (NPSHa insufficiency):
It is the most common type of cavitation and covers about 70% of cavities. To prevent this type of cavitation, the amount of NPSHa in the system should be greater than the amount of NPSHr of the minimum energy required by the pump provided by the manufacturer with curves along with the pump catalog. To prevent damage caused by this type of cavitation, the following solutions Turns.
1- Temperature reduction, which reduces the amount of head due to fluid vapor pressure. When you want to pump a fluid with a high temperature, you must be very careful about this type of cavitation.
Increased fluid level in the suction tank, which increases the amount of static head.
Improve and modify the pump including the following:
3-1- Speed reduction which reduces the amount of Hf (head due to drop).
3-2-Increasing the diameter of the blade spring
3-3- Using two smaller pumps in parallel, which reduces the off-head.
In this case, the liquid is forced to return from the high pressure area of the pump to the low pressure area across the blade. When a vortex current is created in the suction or discharge section, which is due to the high velocity of the fluid, the fluid flow is reversed and rotates in the opposite direction of the normal flow motion of the fluid.
Fluid circulation reduces the useful diameter of the fluid passing through the suction and discharge sections and reduces the fluid pressure (according to the Bernoulli principle). Cavitation occurs as the pressure decreases and the pressure reaches the vapor pressure of the fluid.
This type of cavitation occurs in two ways:
The first is that the liquid inside the pump chamber circulates at the speed of the open engine and suddenly its temperature rises and overheats. Cavitation damage in open pumps occurs mostly at the edge of the impeller blades towards the blade and at the tip of the blades up to the outer diameter of the impeller. In pumps with closed impeller, these damages are caused on the wear strips between the blade and the body of the chamber. Closed pumps can not correct the situation, but it is necessary to release the current trapped in the drain section of the pump. The free space between the tip of the blade and the tongue should be equal to 4% of the blade diameter. Damage caused by this type of cavitation is mostly seen on the tip of the outer blades of the blade and behind the tongue, on the wall of the inner chamber.
Suction type cavitation:
Air suction can occur in various forms in the pipes and other parts of the pump. For example, if a vacuum is created in the pump, air can enter the pipes. One of these examples is the Lift pump. Air can enter the pump in the following ways:
1- Pump shaft seal
2- Leg seal connected to the valve plate in the suction pipe
3- Suction pipes connection rings
4- Flange plate sealing washers in pipe fittings
5- O-rings and screw connections in the suction part
6- O-rings and secondary seals in single seals
7- Single mechanical sealing surfaces
8- Through air bubbles and holes in the suction pipe
9- Foam through liquids
Ways to prevent cavitation of air suction type:
1- Sealing and closing all surfaces, flanges and washers
2- Sealing and closing the sealing rings and stem seals connected to the valve plate in the suction pipe
3- Maintaining the fluid velocity at 8 feet per second (by increasing the pipe diameter)
4- Use of double mechanical seals
A new way to detect cavitation in pumps:
The IREQ Institute in Canada has introduced a new method that is carefully studied by studying the vibration axis of the phenomenon. Principles and mechanisms of this method are based on the processing of vibrational signals of the turbine axis, which are measured by vibration sensors installed on bearings. By processing these signals, the location and intensity of cavitation can be determined with appropriate accuracy.
The advantages of this method are:
1- Accurate detection and location of cavitation
2. The operating condition under which the wear phenomenon occurs is precisely determined.
3- It is possible to apply preventive measures.
4. Periodic inspection is not required.
5. No need for expensive repairs.
6. Performance losses are reduced.