Sand and Gravel Dredge Pumps: Things You May Want to Know

With the increase in the application of mechanical seals, the problem of leakage has attracted more and more attention. The operation of mechanical seals directly affects the normal operation of the slurry pump. The summary and analysis are as follows.
 
1. Periodic leakage
(1) The axial movement of the pump rotor is large, and the interference between the auxiliary seal and the shaft is large, and the rotary ring cannot move flexibly on the shaft. After the pump is turned over and the rotary and stationary rings are worn, the displacement cannot be compensated.
Solution: When assembling the mechanical seal, the axial movement of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate. While ensuring the radial seal, the rotary ring can be flexibly moved on the shaft after assembly. (Press the rotary ring to the spring and it can bounce back freely).
 
(2) Insufficient lubrication of the sealing surface causes dry friction or roughness on the sealing end surface.
Solution:
Horizontal slurry pump: Sufficient cooling water should be provided.Submersible sewage pump: The height of the lubricating oil surface in the oil chamber should be higher than the sealing surface of the dynamic and static rings.
 
(3) The rotor vibrates periodically. The reason is that the misalignment of the stator and the upper and lower end caps or the imbalance of the impeller and the main shaft, cavitation or bearing damage (wear) will be caused. This situation will shorten the seal life and cause leakage.
Solution: The above problem can be corrected according to the maintenance standard.
 
2. Leakage due to pressure
(1) Mechanical seal leakage caused by high pressure and pressure waves. When the spring specific pressure and total specific pressure design are too large and the pressure in the seal cavity exceeds 3 MPa, the specific pressure of the seal end face will be too large, the liquid film will be difficult to form, and the seal end face will be severely worn. , The heat generation increases, causing thermal deformation of the sealing surface.
Solution: When assembling the mechanical seal, the spring compression must be carried out in accordance with the regulations, and it is not allowed to be too large or too small. Measures should be taken for the mechanical seal under high pressure conditions. In order to make the end face force reasonable and minimize deformation, materials with high compressive strength such as cemented carbide and ceramic can be used, and cooling and lubrication measures should be strengthened, and driving transmission methods such as keys and pins can be selected.
(2) Mechanical seal leakage caused by vacuum operation. During the start and stop of the pump, due to the blockage of the pump inlet and the gas contained in the pumped medium, it may cause negative pressure in the sealed cavity. If there is negative pressure in the sealed cavity, Dry friction on the seal end surface will be caused, which will also cause air leakage (water) in the built-in mechanical seal. The difference between vacuum seal and positive pressure seal is the difference in the directionality of the sealing object, and the mechanical seal also has its adaptability in one direction.
Solution: Adopt double end face mechanical seal, which helps to improve lubrication conditions and improve sealing performance. (Note that the horizontal slurry pump generally does not have this problem after the plugging of the pump inlet)
 
3. Mechanical seal leakage caused by other problems
There are still unreasonable places in the design, selection, and installation of mechanical seals.
(1) The compression of the spring must be carried out in accordance with the regulations. Excessive or too small is not allowed. The error is ±2mm. Excessive compression will increase the specific pressure of the end face, and excessive frictional heat will cause thermal deformation of the sealing surface and accelerate end face wear. If the compression is too small, if the specific pressure of the static and dynamic ring end faces is insufficient, the seal cannot be performed.
(2) The end surface of the shaft (or sleeve) where the moving ring seal ring is installed and the end surface of the sealing gland (or housing) where the static ring seal ring is installed should be chamfered and trimmed to avoid damage to the moving and static ring seal rings during assembly.
 
4. Leakage caused by the medium
(1) After disassembly of most mechanical seals under corrosion or high temperature conditions, the auxiliary seals of the stationary ring and the movable ring are inelastic, and some have rotted, causing a large amount of leakage of the mechanical seal and even the phenomenon of shaft grinding. Due to the corrosive effect of high temperature, weak acid and weak alkali in the sewage on the static ring and the auxiliary rubber seal of the moving ring, the mechanical leakage is too large. The material of the moving and static ring rubber sealing ring is nitrile-40, which is not resistant to high temperature. It is not resistant to acid and alkali, and it is easy to corrode when the sewage is acidic and alkaline.
Solution: For corrosive media, the rubber parts should be fluorine rubber resistant to high temperature, weak acid and weak alkali.
(2) Mechanical seal leakage caused by solid particles and impurities. If solid particles enter the end face of the seal, it will scratch or accelerate the wear of the end face. The accumulation rate of scale and oil on the shaft (sleeve) surface exceeds the wear rate of the friction pair. As a result, the moving ring cannot compensate for the wear displacement, and the operating life of the hard-to-hard friction pair is longer than that of the hard-to-graphite friction pair, because the solid particles will be embedded in the sealing surface of the graphite sealing ring.
Solution: The mechanical seal of tungsten carbide to tungsten carbide friction pair should be selected in the position where solid particles are easy to enter. ...
The above summarizes the common causes of leakage of mechanical seals. The mechanical seal itself is a kind of high-precision component with high requirements, and has high requirements on design, machining, and assembly quality. When using mechanical seals, various factors of the use of mechanical seals should be analyzed, so that the mechanical seals are suitable for the technical requirements and medium requirements of various pumps and have sufficient lubrication conditions, so as to ensure the long-term and reliable operation of the seals.

Installation Requirements for ZJ Series Slurry Pumps(1) Pre-Installation ChecksVerify the model, parameters, and component completeness. Review the technical manual before installation.(2) AlignmentFor horizontal pumps, use a two-stage grouting method. Ensure:Centerline alignment with the foundation (±2 mm tolerance).Levelness within 0.1/1000.Coupling Alignment Methods:Straightedge and Feeler Gauge: Max. misalignment ≤ 0.10 mm.Dial Indicator: Runout ≤ 0.15 mm; gap variance ≤ 0.10 mm.For belt-driven units, maintain parallel shaft alignment via pulley adjustment.(3) Piping ConfigurationInlet Pipe: Match or slightly exceed pump inlet size to prevent cavitation/sedimentation. Include a gate valve and expansion joint.Outlet Pipe: Match pump outlet size. Install a gate valve and pressure gauge upstream of the first valve.Key Rule: Throttle flow only at the outlet—never the inlet—to avoid cavitation.(4) Seal/Cooling Water LinesFor mechanical seals, connect water lines per manual (pressure gauge side = inlet).Grease-lubricated bearings require no cooling water.

A03
Nickel Hard 1 - Martensitic White Iron
Alloy A03 is a martensitic white iron that provides reasonable performance under mildly corrosive working conditions and operates at low impact levels. It is typically heat treated to reduce or eliminate residual austenite in the matrix. This alloy is sensitive to slice thickness, and the composition must be adjusted to prevent the formation of undesirable phases.
A04
ULTRACHROME - 24% Cr Corrosion-Resistant White Iron
Alloy A04 is a white iron with a hardness of 375 BHN in the annealed condition. This low hardness makes A04 easier to machine than Alloy A05. The alloy can subsequently be hardened to increase wear resistance. A04 is not as corrosion-resistant as A05 and A12 and is typically not corrosion-resistant.
A05
ULTRACHROME - 27% Cr Corrosion-Resistant White Iron
Alloy A05 is a wear-resistant white iron with excellent performance under corrosive conditions. This alloy can be effectively used for various types of slurries. The high wear resistance of Alloy A05 is provided by the hard carbides present in its microstructure. Alloy A05 is particularly suitable for applications requiring corrosion resistance and erosion resistance.
A06
Nickel-Hard 4 Martensitic White Iron
Martensitic wear-resistant alloy
A07
15/3 Chromium/Molybdenum Iron - Chromium/Molybdenum
Martensitic white iron with slow erosion resistance
A12
HYPERCHROME - 30% Chromium Hyper-eutectic Chromium White Iron
HYPERCHROME alloy is a hypereutectic white iron suitable for high wear resistance, where corrosion is not a concern. It should be used in applications where A05 and A03 cannot provide sufficient wear life. Alloy A12 can be used in mild alkaline slurries with a pH value between 8 and 14. In some severe applications, the alloy may provide up to three times the wear life of A05 and A03 parts.
A25
Nickel-Chromium-Molybdenum Steel - Cast Steel
Alloy A25 is an alloy steel with moderate wear resistance and high mechanical properties. This alloy is used for large castings where toughness is critical.
A49
ULTRACHROME - 28% Chromium, Low Carbon, High Chromium, Low Carbon White Iron
Alloy A49 is a corrosion-resistant white iron suitable for low pH corrosion where corrosive wear is also an issue. This alloy is particularly suitable for flue gas desulfurization (FGD) and other corrosive applications where the pH is below 4. It can also be used in other mildly acidic environments. A49 has corrosion resistance similar to Ni-Hard 1.
A51
ULTRACHROME - 36% Cr, erosion/corrosion white iron, low C ULTRACHROME
A51 is a high-quality erosion/corrosion alloy suitable for applications requiring excellent corrosion resistance and erosion resistance. Compared to alloy A49, this alloy offers significantly improved corrosion resistance, with erosion resistance similar to nickel-hard alloy iron. This alloy is suitable for phosphoric acid loads, FGD loads, sulfuric acid, and other moderately corrosive applications.
C02
Nickel corrosion inhibitor (copper-free) corrosion-resistant cast iron
C21
13% chromium steel - martensitic stainless steel
C23
316-type stainless steel - austenitic stainless steel
C25
Alloy 20 - austenitic stainless steel
C26
26Cr5Ni stainless steel (CD-4M Cu) - duplex stainless steel
C27
'825' Alloy - Austenitic Corrosion-Resistant Alloy
C30
27Cr 31 Ni Stainless Steel - Austenitic Stainless Steel
C44
440C Stainless Steel - Hardened Stainless Steel
C55
Ferralium255 - Duplex Austenitic/Ferritic Stainless Steel
D21
Ductile Iron (SG Iron) - Cast Iron
Alloy D21 is a ductile grade gray cast iron, requiring higher physical properties and greater impact resistance compared to G01.
G01
Cast iron - Cast iron
Standard gray iron
N02
63Ni30Cu alloy - Corrosion-resistant alloy

Translated with DeepL.com (free version)

About Slurry pump test run:

The pump-engine unit can be tested right after it is installed and adjusted. Users with conditions should first test run the pump with clean water, and after a while of normal operation, transport mixture of water and solids.

1.1. Start

1) Open the shaft seal water and adjust the pressure to the specified value;

2) Fully open the inlet valve;

3) Open the water injection valve to inject water into the pump. (Backflow pump does not need water injection);

4) Adjust the outlet valve opening to 1/4;

5) Start the pump-engine unit. After the speed is normal, open the outlet pressure gauge. If the pressure is normal and stable, slowly open the outlet valve till it is fully opened or meets the working conditions.

Note: Starting the pump with the outlet valve fully open will cause motor overload; using a small inlet valve to control the flow will cause pump cavitation, both of which should be avoided.



1.2. Operation

After normal operation, the following items should be observed:

Whether the flow rate and head (inlet and outlet pressure) of the pump are stable and meet the process requirements; whether the current is stable; whether the unit has abnormal sounds and whether the noise is too loud; whether the shaft seal leakage is normal (drip seepage);

Mechanical seal size ≤ 50mm, leakage ≤ 3ml/h. Mechanical seal size > 50mm, leakage ≤ 5ml/h. Bearing temperature rise 35°C, the maximum temperature shall not exceed 75°C;

Vibration value should be kept within the specified range.



1.3. Stop the pump

① Before stopping the pump, pump clean water for 30 minutes to clean the medium inside the pump and pipe.

② Close the pump outlet valve.

③ Shut down and close the corresponding shaft seal water and cooling water.

④ Close the pump inlet valve.

Note: It is never allowed to stop the pump when the outlet valve is fully open, otherwise it is easy to form water hammer, causing damage to the pump or pipe fittings.