Sundyne 331 parallel operation problem 2

[igauravg]

Dear All,

We are using 331 model for PTA slurry service, 3 pumps run in parallel and one stand by
rated flow - 37.5 m3/hr, differential head- 950 mtr

I last 4-5 months around 4-5 gearbox have failed,in one input shaft gear sheaved, in 3 output shaft gear sheaved

Our diff users are eroded
We have common control valve for controling, and individual pad type pg

These PG are showing diffrent pressure, diffrence is about 15 bar, how can this be possible in parallel service,or is this the indication of our problem
kindly suggest some sollution

 

[JJPellin]

I would not recommend running these pumps in parallel. You did not provide enough information to evaluate the problem. It should not be possible for the pumps to have different discharge pressures. How are the gears failing? What is the best efficiency point for these pumps? Where is the errosion damage? What are the suction conditions?

Johnny Pellin

 

[GHartmann]

What I remember about the Sundyne pumps is that they have a falling head curve as the pump approaches zero flow. Thus at the curve apex the pump can go either to low or high flow. This makes the pump unstable (possible leading to some of your damage). I want to recall that each pump needed to have its own flow control system. I do not think an AutoMatic recirculation valve (LARC or equivalent) will work because they are based on pressure.

The high impeller speeds of the pumps in an unstable area make the pumps more susceptible to damage.

Each pump therefore needs to be controlled in the high flow region beyond the head curve apex.

Can you get a Sundyne applications expert to help you out?

I hope the above is understandable and helps steer you in the right direction.

You might Google or do a Yahoo search on "pumps in parallel operation" and see what turns up.

Gerry Hartmann
BS ChE NC State University

 

[GHartmann]

I have attached a pump installation guide from Sundyne. Take a look at pages 14 - 19.

Hopefully this explains the problem better than I did. If you truly have 3 pumps operating with one standby, the problem becomes even worse.

To solve the problem is another issue. Adding a separate flow control system to each pump could be quite a repiping effort to your installation. Adding a "kick-back" line with an orifice limits the ultimate flow delivered by your pumping system and may not be adequate to meet the process demands (probably 10 to 15% minimum will always be recycled; it could be more). Thus you lose this pumping capacity.

The ultimate solution might be replacing the pumps with a single operating pump that provides the needed capacity. (You can have an installed spare). Be sure to take into account the minimum flow requirements or flow control system to keep the pumps in their safe operating zone.

Keep us posted on what you find.

G.Hartmann
BS ChE NC State University

 

[GHartmann]

igauravg:

I have reread the Sundyne brochure and towards the end the brochure describes exactly the mechanical problems you are having as a result of operating in the low flow area of the pump operating curve.

You will need to analyze the demands of the system, especially during periods of low flow requirements of the process. During these periods your flow control system will force the pump back up its curve to the point of instability. Once at this point your parallel pumps can become unbalanced (i.e. one pump 90% of total flow the other 10%).

One other possibility to investigate is reducing the head requirements of the system resistance line (removing system pressure drop, resizing control valve and control elements, etc).

Sorry if I have beat your problem like the old horse.

G.Hartmann
BS ChE NC State University

 

[JJPellin]

Ghartmann has given you a lot of good advice. These tend to be high speed, high energy pumps with poorly shaped curves. They are very sensitive to operation at excessively low or excessively high flow. The best option is to not run them in parallel. But, for your installation, this is probably not an option. The next best option is to measure and control the flow from each pump individually. Using our definitions, we would no longer consider them to be run running in parallel if they had individual flow measurement and individual controls. The third best option would be to have individual flow measurements with appropriate alarms for high or low flow. The forth best option would be to set conservative limits on the total flow to try and keep the individual pumps out of trouble. This can be difficult with only two pumps running in parallel. With three running pumps, this can be almost impossible.

I would set limits on the total flow from the group of pumps so that they were running between 80 % and 105 % of the best efficiency point flow for the group. This would probably require a minimum flow spill-back line with appropriate controls. I would insist on running each pump individually at least once per year to take performance data to look for degraded pumps.

You have provided us with so little information that we are really just guessing that low flow operation is your problem. Your gear issues could be related to lubrication. Your erosion problem could be a materials issue. Unless you tell us more, the best we can do is guess.


Johnny Pellin

 

[igauravg]

Thanks all for providing your suggestion

Our process demand will always in the range of 115- 127 m3/hr

Secondly these pumps are handling slurry

In actual installation there are 3 pumps 2 runs in parallel and one in standby but later to increase plant capacity one more pump has been added

we dont have individual control or kickback line, what we have is common suction pressure,currents, pressure and common flow

We are also exploring the possibility of installing HMV model

Our piping is also problamatic, pumps are installed one by one and suction pipe is from one side

Our diffusers are heavily eroded(may be by high velocity PTA slurry)so might be internal circulation of flow is creating vibration.

We are also in contact with sundyne personal regarding this problem

Kindly ask me if other information required

 

[GHartmann]

Igauravg:

Some more things to think about:

Have these pumps acted this way from the beginning?
If not, what has changed with the installation or process conditions/operations?
Have you increased the flow without changing the suction piping?
Could the PTA slurry have increased suction or discharge friction losses due to build-up of solids in line?
Could the diffuser damage be corrosion? (I understand PTA slurry may also be corrosive to 316 / 316L)?

More specific to your installation:

Are you positive the demand is ALWAYS 115 - 127 m3/h? Certainly there are times when the process flow is less (start-up or downstream demand changes).
What happens to the pumps when there is zero demand?
I assume the pumps are at a common suction pressure.
When monitoring the pumps do they consume the relatively same amount of power (same current for each pump)?
Do you really have no kick back line at all? (This is not ideal for these pumps). Or do you mean you do not have a kickback line for each pump?
If I understand your suction arrangement (line coming from side) the first pump in line could rob the second pump of flow causing a low flow situation in the second pump (starting unstable operations again).

The Sundyne pumps are more susceptible to overpowering each other in parallel operation than are other centrifugal pumps (due to head curve decreasing towards shut-off).

I would be extremely cautious about just changing to the HMV. It has the same head characteristics of the LMV and could experience the same difficulties.

As JJPellin stated we have done as much as we can from a remote location.

Hopefully the Sundyne applications expert can help.

We would like to hear how it turns out.

G.Hartmann
BS ChE NC State University