Sundyne Internal Pump Pressure

[mazamm]

Hi everyone!!
Before going through my problem, kindly check the lube oil circuit of Sundyne pump attached. We have been facing a problem with a internally shaft driven PD pump whose pressure drops with time from 3 bar. As the pressure reaches around 2 bar we have to start auxiliary pump as well. The another problem which is more concerning to me is that when we start the auxiliary pump, the pressure does not increase that time. After few minutes or may be few hours, the pressure gage shows the pressure around 5 bar. Why there is a delay in this pressure rise ?
Apart from leakage through connectors or check valve passing, what can be the other factors contributing towards pressure drop of internal pump ?

Regards

 

[bingopin]

Excessive high speed shaft bearing clearance can cause low lube oil pressure. This is usually a sleeve bearing where the input and intermediate shaft bearings are ball bearings. The difference between cold oil pressure and operating temp pressure would explain the drop in pressures due to lower viscosity. Don't know what type of oil you are using. I also don't know what oil Sundyne now recommends but they used to recommend ATF due to its relatively flat temp/viscosity curve.

Also check each lube point for the restriction orifice (called "jets" on the drawing). Look carefully at the internal pump relief valve. This is usually a spring and lapped plate arrangement integral to the pump.

 

[JJPellin]

There are a number of possible causes. These are the first twelve that come to mind.

[ul]
[li]As noted, the bearing clearance in the high speed bearings on the output shaft could be increased.[/li]
[li]There is a spring between the end of the input shaft and the top of the internal oil pump. If this spring was weak, it would allow pressure to relieve back into the sump.[/li]
[li]The main oil pump is above the oil level. It draws suction up a piece of tubing. If there are any leaks in the suction tube, air could be drawn into the main oil pump, reducing its capacity.[/li]
[li]For some models, there is a separate internal relief valve. It could be leaking and relieving at a lower pressure.[/li]
[li]There are a number of o-rings that seal between the three sections of the gear-case that could be leaking.[/li]
[li]The oil filter could be plugged up. The standard Sundyne filter is equipped with an internal relief that bypasses the filter if it plugs off. That could be relieving.[/li]
[li]The main oil pump does not have a check valve. If the main pump was worn out, the discharge of the auxiliary pump could flow backwards through the main.[/li]
[li]The internal oil pump is driven by pins from the end of the input shaft. Another pin positions the bottom of the pump relative to the center housing of the gear-case. If these pins were distorted or damaged, the passages of the pump may not line up properly to the ports in the housing.[/li]
[li]If your system is equipped with an external cooler, it could be fouled or plugged up on the oil side.[/li]
[li]The oil filter housing has an internal plug that forces the flow out through the external cooler (if so equipped). If that plug is installed with no external cooler, the flow path would be blocked.[/li]
[li]There are threaded plugs, drilled with orifices that feed the oil to each ball bearing individually. Those plugs could be missing (left out) or leaking by badly.[/li]
[li]The wrong oil could be used.[/li]
[li][/li]
[/ul]
Given your description, I would suggest taking the gearbox apart and checking all of these.


Johnny Pellin

 

[mazamm]

@bingopin: All of the bearings are journal bearings. But how come the high bearing clearance can result in low lube oil pressure ? Moreover, I have been taking the readings at the temperature gauge and found no difference in temperature during this drop eliminating the possibility of pressure drop due to temperature / change in viscosity.

 

[mazamm]

@bingopin: Sorry both the bearings of high speed shaft are journal bearings.

 

[mazamm]

@Johnny Pellin:
1) I will look for the spring of relief valve during gearbox overhaul and will check by pressure test whether the relief valve passes or not.
2) I doubt the possibility of suction tube leakage since the pressure holds at 3 bar for quite a long time and then after few hours it starts to drop.
3) It is equipped with an external shell and tube oil cooler. But if it gets fouled, the back pressure will probably increase since it is a PD pump and has to force the desired capacity out of the pump. So reduced pressure may not be due to this problem.

 

[mazamm]

Can someone put light on the second part of the question that why there is a delay in pressure rise as the auxiliary pump starts ?

 

[JJPellin]

You did not mention where you are reading the oil pressure. Normally, this is on the upper gear-case at an oil galley that feeds the upper ball bearings. If so, this is downstream of the filter and cooler and so, blockage in either of these could still match your circumstances. A leaky suction tube to the main oil pump could change performance as the oil heats up. But, it would be unlikely to affect the auxiliary oil pump performance. The pin that positions the internal oil pump to the center section of the gear-case has a particular height requirement. If it is too tall, it will hold up the internal pump, creating a leak path that would affect both oil pumps since there is no check valve on the internal pump. The auxiliary oil pump could be piped incorrectly and not getting proper prime.

The oil feed into the high speed journal bearings is restricted by the bearing clearance. If the bearings are wiped, the oil pressure will drop, but would probably not be so variable. You did not mention the service for the pump. If the process conditions are changing over time the thrust loads on the high speed shaft will change. If the thrust bearings are failed, the change in thrust load could result in a change in thrust position which can also change the oil flow through the high speed bearings.

It is hard to say how these possible problems would account for the changes in oil pressure over time. But, if you plan to pull the gearbox apart for repair and inspection, it is easy to inspect for all of these conditions.


Johnny Pellin

 

[mazamm]

@Johny Pellin:
You are right with respect to pressure gauge. As far as viscosity and temperatures are concerned, I have already mentioned that the sump temperature remains constant at temperature gauge. Do you want to say that a leaky suction tube will reduce suction pressure and correspondingly the discharge pressure ?
The process is actually a Slurry liquid with discharge pressure maintained at 25 bar. I don't know how variation in thrust load can be checked keeping in mind that the discharge pressure is maintained.

Can you put light on the second part of the question that why there is a delay in pressure rise as the auxiliary pump starts ?

 

[JJPellin]

I would think the most likely causes would be associated with a plugged oil filter, incorrect oil or failure of the internal relief valve. A leak in the suction tube to the main oil pump would allow air bubbles to enter the main oil pump. It is a gear pump (positive displacement). The introduction of air bubbles would reduce the capacity of the pump. The air bubbles take up space that is no longer available for pumping oil. A reduction in pump capacity will result in a reduction in system pressure. This is the nature of the system curve, not a characteristic of the pump.

The oil could be foaming excessively. This is a common problem with these gearboxes. If the foam gets severe enough, you could be drawing foam into the oil pumps, once again affecting the performance of the pumps and the pressure in the system. This could take hours to develop. What oil are you using? Do you see excessive foaming in the sight glass?

No. I cannot address how each of these possible causes would result in the exact change in pressure that you are seeing over time. You describe everything as nearly steady state (process conditions, oil conditions). What causes changes in a steady state system? I don’t know. But, in the end, it does not matter. If you are taking the gearbox down and opening it up anyway, I would suggest that you check everything. If you don’t find anything wrong, I would suggest that you replace the internal oil pump. They are small and relatively inexpensive.


Johnny Pellin

 

[JJPellin]

If you find the problem when you overhaul the gearbox, please post your findings so that we can all learn from your experience.

Johnny Pellin