Improvement and Application of Centrifugal Pump Seal Structure

1 Introduction Yanhua refinery 100Y-type oil pump, the original seal between the bearing housing gland and shaft seal structure for the labyrinth seal, plus a dust ring in the labyrinth seal, the sealing performance is poor. As the pump speed is high, under the action of centrifugal force, the oil in the bearing box along the shaft thrown from the end cap, oil leakage is very common, resulting in bearing boxes, couplings and the surrounding ground covered with oil, to the safety of production Bring about the hidden danger, cause unnecessary waste of lubricating oil at the same time, have polluted the production environment seriously, affect the civilized production and enterprise's standard upgrade. If the operator can not find in time, there may be damage to the pump. Therefore, it is very necessary to improve the structure of crude oil seal and develop a new type of oil seal structure with excellent sealing performance and long service life.

2 original seal structure problems Pump operation due to the oil level limit, the lower end of the nut immersed in the oil pool, the oil stirred up, drip along the inner surface of the gland, drip directly to the rotating shaft, a lot of oil accumulation on the shaft, Driven by the rotating shaft, the oil crawls along the shaft and enters the gap between the gland and the shaft labyrinth seal. Because the labyrinth seal gap is large and the seal resistance is small, once the oil is difficult to get back into the labyrinth, it constantly thrushes out of the shaft. The formation of oil accumulation at the end, the labyrinth and dust ring can not play a sealing effect, that is, oil spills. From the above analysis we can see that the main problem of the crude oil seal structure is that the bearing lock nut is too big and the labyrinth sealing gap is too big to be effective sealing effect.
3 to improve the feasibility of the program and the initial selection of a new leak-proof oil structure should meet the following requirements: First of all to ensure that the sealing part of the combination. Meanwhile, the structure is compact, the system is simple, the manufacture and maintenance are easy to use, the finished product is low, the work is reliable, and the service life is long. The main reason for the oil spill, should start with the following two aspects to improve to prevent oil spills.
(1), to reduce the oil thrown oil bearing lock nut immersion depth (the level and the lock nut relative distance) is the size of an important factor affecting the amount of oil thrown. Reduce the nut diameter, can reduce the oil immersion depth, reduce oil mixing, thereby reducing the amount of oil fell on the shaft, indirectly reducing oil leakage.
(2) Improve the structure of crude oil seal structure analysis, from the seal point of view can consider the principle of packing or blocking, separation, lead or injection and flow resistance, reverse transport, and combinations of these programs and other methods. First of all, you can take a mechanical seal. According to mechanical seal performance, scope of application, life expectancy, mechanical seal can be applied, but the mechanical seal price is high, complex structure, the required space, disassembly inconvenience, not suitable for such a small space structure. Second, consider the use of lead-out or injection method to achieve the sealing requirements, but the need for auxiliary devices, the structure is complex, and therefore undesirable. Consider the use of packing and blocking methods, because of the longer life expectancy, some contact seals such as felt ring, gear ring, seals, seals and other contact with the shaft wear and tear, limited life expectancy, not suitable for high-speed long-cycle operation, and easy Axis occurs, and thus ruled out. Finally consider the use of flow resistance or reverse transport, or the use of integrated programs. Flow resistance is the fluid resistance required to seal with a narrow gap or tortuous path of the seal. Reverse transport is the use of seals on the back of the fluid caused by back pressure, so that part of the balance or complete balance of the fluid reverse input to the upstream, in order to achieve the purpose of sealing. It is characterized by no mechanical friction, compact structure. Reverse fluid (also known as dynamic pressure), including labyrinth spiral seal, dynamic seal, screw seal. Taking into account the small space, long life, low power consumption, simple structure, easy disassembly, low price requirements, the use of flow resistance or reverse transport and its comprehensive program optimal. In the case of flow resistance or transfusion solutions, helical seals are the most capable of meeting the above requirements, so a spiral seal is preferred. As the centrifugal seal can be used in conjunction with other seals, in order to ensure the effect of dense, select the combination of spiral seal and centrifugal seal. As shown in Figure 1:
4 For 100Y pump design calculations:
According to the theoretical analysis, the helical seal helix angle α has the lowest power at 5 ° 6 ', and the maximum seal pressure at 15 ° 39' with the same length. Based on some data, practical experience is given, taking into account the sealing pressure, power consumption and structure length, And leave some margin of safety, the design is as follows: [1], [2]
4.1 spiral seal design and calculation of parameters (1), helix angle α
For the 100Y pump, the shaft end diameter d = 65mm, the length of the outer shaft can be spiral seal L0 = 56.5mm, the shaft end associated with the coupling, leaving 7mm interval, so take the seal length L = 39.5mm. Since the length and diameter are small, the seal pressure formula (see below) [1], the sealing pressure is small, so the larger the helix angle selected as α = 5 ° 49 '. Then the tangent of α t = tan α = 0.1019 (this value is calculated later).
(2), the relative groove width u generally take u = 0.5 ~ 1.0, where u = a / (a ​​+ b) = 0.75.
(3), the relative groove depth Ï… generally take Ï… = 2 ~ 10, taken as Ï… = (c + h) / c = 5.
(4), the sealing clearance c recommended c = (0.6 ~ 2.6) / 10000m take c = 0.26mm, because the centrifugal seal before the spiral seal can ensure the sealing performance, take the big gap in order to prevent errors due to processing and installation errors, and Shaft friction.
(5), the groove depth h, h is calculated by the formula h = c * (Ï…-1): h = 0.26 * (5-1) = 1.04, h = 1.0mm.
(6) The number of heads i, the number of heads i The selection principle according to the number of heads discussed above: high speed (n> 5000r / min), the first choice of the head; low speed (n <5000rmin) choose the long. As the pump n = 2950r / min, according to the information recommended, take i = 4.
(7), spiral lead s, take s = 16 (rounded to facilitate processing);
s = πd * tan α, α = 5 ° 49 '.
(8), spiral groove width a, tooth width b
By the formula a = πu * d * tan α / i = 3.00mm
b = π * (1-u) * d * tan α / i = 1.0 mm
(9), the angular velocity of the shaft ω and the spiral circumferential speed v calculation:
ω = 2πn / 60 = 308.9 rad / s
v = πn / 60 = 7.6 m / s
(10), the length of the helical structure is L = 39.5mm
(11), screw seal pressure Δp ', Δp' = rωw d LCp / c2 [1]
tu (1-u) (υ-1) (ν3-1)
Where: Cp = -----------------------------------
(1 + t2) υ3 + t2u (1-u) (ν3-1) 2
Substitution of u = 0.75, t = 0.1019, υ = 5.0 yields: Cp = 0.178, according to Cp = 0.178, the coefficient of viscosity of thin oil lubrication is small μ = 0.00223 × 9.8Pa / s, ω = 308.9rad / s, d = 0.065 m, L = 0.0395 m, c = 2.6 × 10 -4
m into the calculated Δ p `= 35800Pa.
(12), spiral seal power calculation N [1]
N = πω2d3LCn / 4c
Cn is found by the formula: Cn = 0.46,
Then N = 14 watts pump motor rated power of 90KW, shaft power 75KW, spiral seal power consumption of 14W, we can see the power consumption is small, the original pump does not affect the operation.
4.2 Calculation of centrifugal impeller seal The differential pressure generated by the impeller centrifugal force ΔP "formula is: [1]
ΔP "= k2ω2 (R22-R12) r / 2
= 0.52 × 308.92 × (0.0352-0.0252) × 800/2
= 6403 Pa
Where: R1, R2 -------- respectively, the diameter of the impeller, the outer diameter of the gas-liquid interface radius
k ------------ Coefficient, approximated by a smooth disc, k = 0.5
4.3 combined seal of the screw seal and the centrifugal seal The total pressure difference combined seal pressure difference ΔP is the sum of the screw seal pressure difference ΔP 'and the centrifugal seal pressure difference ΔP ", namely: ΔP = ΔP' + ΔP" = 35800 + 6403 = 42203 Pa
Because the bearing box is basically atmospheric pressure, the pressure inside the box is similar to the atmospheric pressure outside the bearing box, so the combined seal pressure difference ΔP is to prevent the oil seal pressure outside the string, the pressure of the seal structure 0.04MPa, fully consistent with should be slightly higher than Machine pressure requirements.
4.4 Material Selection As the bearing gland can not afford to bear the pressure, the optional A3 steel, oil thrown impeller not only from oil and oil resistance, but also bearing ring, in order to prevent damage caused by multiple disassembly, 45 steel can be used .
5 Application After the oil seal was modified, the trial run was carried out on the ketone-benzene workshop pump 305 (model 100Y-120 × 2) in April 1998, and then on the pump 430, pump 351 and other pump. After more than four years of operation, the bearing box gland seal without any leakage, fully meet the design requirements. Prior to this, the original seal structure due to frequent leaks, the Secretary pump station operators a lot of work in the hourly inspection of the pump oil to be supplemented, but also occurred in two due to oil spills Axle equipment accidents, resulting in fluctuations in production, as a result of the universality of oil spills also make the scene of health has been difficult to solve, to the scene management of the equipment has brought the difficulty. After the transformation, during the oil change period, the pump oil basically does not need to be replenished in a large amount, greatly reducing the labor intensity of the workers and obviously improving the site conditions, which also creates favorable conditions for smooth operation of the production.
6 Conclusion The original bearing housing gland labyrinth seal instead of centrifugal seal and screw seal combination seal structure, the use of centrifugal seal as a preliminary throttle blocking seal, the use of spiral seal as a secondary seal, you can ensure that the shaft without any leakage . This improved method only need to transform the bearing lock nut and bearing box gland, the original structure of small changes, that can meet the sealing requirements, but also to adapt to the original structure of the available space constraints. This kind of sealing structure is simple, easy to manufacture and install, and consumes less power. From the actual use of the results, the original structure of the equipment and the surrounding environment has been completely improved by oil pollution situation, eliminating the production risks; reduce labor intensity and promote civilized production. In addition, by adjusting the structural parameters, the structure is also successfully applied to 65Y, 150Y-type pump, also achieved satisfactory results. Therefore, the structure can also be applied to other machine pumps with similar problems, which have great promotion value.
references:
Gu Yongquan. Fluid Seal. China Petrochemical Press, 1990, NO.85-120
2 Hu Guozhen. Chemical seal technology. Chemical Industry Press, 1990, NO.464-483

About the Author:
Yin Zhigang graduated from Chemical Engineering Department of Chengdu University of Science and Technology in 1997. Now Beijing Yanshan Branch refinery ketone benzene workshop engaged in equipment management and maintenance work, engineers.

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