Design Guidelines: Sourcing Suggestions for 1500-GPM Hydraulic Diffuser

Hi Zhock

There are a few things that come to mind with this post:-

1) The function of the diffuser you speak of is just to reduce the velocity (and turbulence) of the returning oil. What you need is a (very) large return line pipe that is conical (progressively becomming larger)! The pipe should be situated about a third the way down the reservoir and be far enough away from a baffle not to re-create turbulence.

2) The static internal baffles of the reservoir should be manufactured so that they come to within a few inches of the normal surface of the oil (don't forget emergency corner diagonal cutouts at bottom). This causes a wear effect that allows air bubbles to expand as the oil is forced upwards and does not allow them to re-integrate into the oil.

3) The inlet and outlet need to be as far apart as possible!

4) Have you got some other problem/situation that currently entrains air in the system?

Good luck with it!

Thanks for you response.

Regarding Point 2

a)Can you clarify what you are suggesting. Typically vertical baffles would extend above the oil level to seperate the different areas.

b)What do you mean emergency corner cut outs? I would typically locate the cutouts near the bottom but not quite at it to help prevent any sediment from being pulled back towards the pump inlet.

c)Can you elaborate on what you describe as the wear effect as the oil is forced upwards please.

Regarding Point 4

There are some entrained air issues. Particularly the high pressure pumps suffer from abbreviated life due to the cavitation like effect of drawing air into the inlet.

Hi Zhock

Firstly, sorry about the spelling. I meant WEIR!

The vertical baffles should NOT rise above surface as the idea is that the oil is forced upwards and over the baffle (the WEIR effect). The reason for this is that, as the oil is forced to rise, so the entrained air bubbles are forced to expand. If the oil is ultimately heading towards a suction pump inlet at low level, the expanded bubbles cannot follow the oil to the low level.

The second point was: the corner cutouts at the bottom of the baffle(s) serve to protect the system from oil starvation should the oil level inadvertently fall below the vertical baffle height. The area of the cutouts should be able to provide minimum oil flow to the pump without causing starvation (not cavitation).

Your last point was lost pump life due to cavitation. Starvation is where a pump can draw air into the inlet (which is obviously a bad situation) through lack of available oil (or a hole in the inlet pipe). Cavitation is completely different in that: due to (generally) bad design, the inlet of the pump creates enough negative pressure to induce the oil to momentarily boil. This creates vapour pockets and will erode the pumping components. It will sound like gravel being drawn through the pump!!!

To counter cavitation, ensure that the pump inlet is flooded (ie below oil level) by a significant amount to reduce negative inlet pressure. Also try to ensure that the inlet pipe is mathematically of large enough bore to provide the required flow at LOW velocity. ALSO ensure that the inlet pipe is as straight as practical for your setup. Sharp bends are instant death if installed on large pump inlets as you require laminar flow.

Good luck with it AND be careful, ten minutes with a flow modelling package (such as Flowmaster) will save you $$$$k and provide all your required answers (often with surprising results)!!!

Thx again.

Re: Vertical Baffles - The emergency cutouts make sense now. The concept of the weir effect makes some good sense. I have not seen it applied in traditional hydraulic applications before. Have you ever applied it this way?

Traditionally the baffles are arrange with what you call the emergency holes positioned to allow the longest path through the tank provided more time for the air to rise. They are low in the tank so presumably let mostly de-airated fluid to move through preferentially.

Suction ports on this application are all ported out the bottom of the tank so will have positive head pressure and adequate port size to prevent true cavitation. The point I make is that with entrained air in oil, high pressure pumps are able to force the air into solution once on the pressure side of the pump causing the resulting implosion and removal of material and damage to pump components without the usual negative inlet pressure being the source of the bubbles.

I have not dealt with Flowmaster berfore. I have just entered the world of Solidworks which has a flow modeling package that I have not yet purchased as I am still learning to walk so to speak. Will investigate flowmaster. Thx.

Hi again

The concept of the weir effect makes some good sense. I have not seen it applied in traditional hydraulic applications before. Have you ever applied it this way?

The norm for vertical baffles is as I have described and, Yes, it is my usual method of design, however, I have also seen many other methods applied. I have also seen baffles either omitted completely or (even worse) welded entirely to the top of the reservoir (forming two separate reservoirs) with disastrous results!

One further thought:- if you should apply my recommendation for the vertical baffles then, wherever the top of the baffle ends up being (at least six inches below full oil level in you very large reservoir) THEN THAT IS WHERE YOU NEED TO INSTALL YOUR LOW LEVEL INDICATOR/ALARM! You do NOT want to rely on the emergency cut outs at the bottom of the baffle to supply sufficient oil (just in case your maintenance guy leaves a swab out rag in the bottom) !

Flowmaster BTW is extremely good and comprehensive but takes awhile to setup for each 'run'! It is also extreeeeeeeemly expensive! If you haven't already got access to it then....... use 'best engineering practices'!

Good luck........you're on the way to a better (stress free) life !

I am working your recommendation into the design in combination with regular baffles to maximize distance from return to suction. Do you use multiple weirs or a single set one flowing over one flowing under? I will be trying to optimize and am trying to visualize/sketch what fits/ makes sense and doesn't cost too much to fab.

Also the emergency cut outs. What sizing formula do you use. If they are sized as I would in tradional way they would allow the oil to pass through and would dramatically reduce the amount going over the weir.

Hi zhock

I must admit to having only (normally) fitted one baffle however, an over and under system might work well although, this would require three baffles to be useful! It depends on the size of your res', it's orientation, surface area for heat loss, decontamination/filtration and a myriad of other considerations.

The cutout size is an interesting question. The formula to use to give the size per flow is A=Q/v where Q= Flowrate, A= cross sectional area and v= fluid velocity. The formula can be manipulated to proved dia if needed or just look on a standard piping nomograph to give a pipe size and use the derived area. HOWEVER, in the past, I have had great success using the 'dry pump' concept! I don't know the type of pump you are utilising but it is possible to run pumps almost dry for long periods (if you can't stop the pump but don't want to generate power loss (heat)! This concept certainly works for a gear pump!

If this is the case, you could consider only providing a lubrication supply via the emergency cutouts (with alarm bells ringing throughout the factory)! If this is not the case, you must provide full flow capacity through the cutouts and rely on a thermal effect to send the flow over the baffle(s).

If you require any reading matter on the subject, I could recommend 'Industrial Hydraulic Control' by Peter Rohner (ISBN 0 9581 493 1 3). Reservoirs are covered in Chapter 9.

I'm sorry if this only serves to confuse you. It's all food for thought though!

I have come up with a design that incorportates your weir concept with a bit of a twist incorporating some horizontal baffles to limit swirling and create vortex break. It is drawn in solid works and would be willing to send it to you if you want. I must admit it is completley different than any baffle configuration I have seen but based on some constraints of return and pump suction locations I believe it is actually pretty cool and maximizes the tank volume,and distance from return to suction. Thanks for your input.

Hi again

It sounds like you are about sorted. I do think that if you are installing horizontal baffles, you should consider 'mewebe's suggestion to use a small mesh material for this. I have seen it used with good results as air bubbles congregate together on it and are expelled when large enough. As said, this method usually calls for the mesh to be placed at 45 degrees but anything on those lines will have a positive effect!

You could post a drawing to my mail box but I can only read AutoCAD at the moment although I am in the process of acquiring Solidworks. I would love to see the finished design.

I hope all the commentators at CR4 have been some help and look forward to you becoming an active contributor very soon!

Sounds like a large hydraulic press. Baffles are cheaper than diffusers, but keep the fluid velocity reasonable to avoid inertial forces on the baffles. You will need to look carefully at the volume of oil returning to avoid pushing aerated oil over dividers, and design a series of underflow weirs and dividers to remove entrained air. The next problem is stirring up dirt from the tank floor, which can cause reliability problems with this type of system. A gauze on the tank floor will help to hold debris in place in combination with the weirs.

It is a large press. Baffles are still fairly expensive as the tank is stainless and I have intended on using 2 already. One being standard on most systems. The return flow on current system is crazy. Only 4" header so velocities are in the pressure range not the return line range. On current system the entire tank is highly agitated on every cycle. The challenge being that after the return flow comes in the cycle repeats in aprox 30seconds so not alot of time to allow air to come to surface.

Gauze on the tank floor sounds interesting. What kind of gauze do you mean?

The most economical way to stop internal turbulence might be an internal diffuser pipe on the tank inlet, ie just a perforated tube. I would divide the tank into a minimum of three sections, and not allow the oil to overflow a weir. Holes towards the bottom of the dividers will allow the less aerated oil to pass and allow longer for the bubbles to rise. In very aerated systems a gauze screen mounted at an angle can help retain bubbles. For a debris trap on the tank floor, I would use say two layers of a 1/4 inch weld mesh. Take care to feed the hot pump case drain a long way from the pump inlet. Don't feed the case drain into a return pipe with pressure spikes! If you want good service life, I would consider a large return accumulator and a fine return filter downstream. This would also calm the tank down.

Good points. I do use accumulators frequently on return lines to minimize spikes. This application is not quite a fit for them. Mesh is a good idea. I have decided to build a scale model and can also scale flow to the tank size and test some of the ideas including the mesh you suggested and the weir. I appreciate everyones input. I have never spent so much time thinking about tank design, kind of take it for granted but think I will continue to investigate and experiement with different configurations including the pancake spiral baffles configuration. Keith Bowers suggested.

Thanks all again.

The tank should be about 2,000 gallons as a minimum and the return line should be less than 5fps submerged by 2ft. I would suggest a verticle cylinder 7ft dia x 9ft high. Mount the tank in containment along with the pump and primary valves. Nothing like no possiblity of an accident. Put a generous insection/cleaning port on the top 3ft dia x 1 ft high. Mount all gauges and safety alarms from the top.

NO!! Not a vertical cylinder--that minimizes surface area from which the air can escape.

A LARGE diameter inlet pipe is first requirement--3 fps or less is desirable. A 12" ID pipe and inlet nozzle is required. The 10" section should be at least 10 feet long to avoid tunneling. The reservoir would preferable be round with a tangential inlet and the outlet in the center of the bottom (with a 1/2" high lip to block debris, lip drilled with 4-1/4 inch weep holes at 90 degree spacing. ) A spiral baffle with at least 100 sq inch flow area, growing out from the center will ensure maximum 'settling time.'

Then ensure the oil is well filtered to keep all fine particles removed (they stabilize foam). The oil must also have anti-foam additive incorporated in the additive package.

Thx for your response. The return oil comes back through a dump valve. System is first decompressed and when at low pressure (aprox 300psi) the dump valve opens fully and the press comes down. Oil out of the dump valve is actually going into the tank currently with a 3" pipe so veocities are crazy. I was going to split the flow into multiple 2" lines 4pcs and plumb with hose to physically seperate/isolate the dump valve from the reservoir as it is also problematic due to mechanical and hydraulic shock issues which are being solved seperatley.

Your ideas sound interesting. Some questions or points I would request clarification on.

1)What do you mean by inlet nozzle?

2)You mention 10" section - 10' long to avoid tunnelling. Please elaborate or expand on what you mean.

3) Your first comment was not a "vertical cylinder" but you then say the reservoir should be round. I am trying to picture the the tank and baffle arrangement that you suggest as it sounds interesting. I take it you are then describing a horizontal cylinder.

4) What do you mean by 4 1/4" weep holes.

Are you describing a tank style that is used in industrial applications today so I can see one and get a better understanding of what you suggest.

As I have replied to others. There are a total of 5 pumps on the system so single point for the outlet in center bottom is problematic.

Interesting point on dirt stabilizing the foam. It is kept clean but will boost filtration a bit. Anti-foam additive would be standard for this oil but will investigate additional additive for this application.

Round, flat--think like a thick pancake--maximum horizontal surface area for air disengagement. Inlet line 10" x 10' if too short, high velocity 'core' never slows down, exits at high velocity.

Multiple pumps no problem--single outlet from tank, and then take off individual suction lines from the single LARGE (6"?) outlet line. NEVER have more penetrations in a tank than absolutely necessary. Much cheaper to add connection to PIPE.

Inlet nozzle is the actual connection to the tank wall. A tangential entry is not a 'standard' fitting, but is necessary in this situation to control excessive agitation/air entrainment. Needed to cause a circular flow of the oil into the 'spiral baffle' to absolutely maximize the residence time in the tank to maximize air disengagement. I suggest this spiral baffle height be from 1/4" from the bottom to 1" from the top. 'Spacing' of the spiral must maintain at least 100 In2 of flow area to reduce turbulence and allow disengagement.

4-1/4' weep holes are typically used to allow any water and final residual oil to slowly drain out when the tank is emptied for cleaning and inspection. Otherwise the 1/2" high dam around the outlet nozzle will hold back lots of oil. Outlet nozzle must be equipped with a standard vortex breaker. Vessel fabricators understand this stuff.

Bigger is better of course. The 1500 Gallons is nearly double the current tank volume. Space is a limiting factor with equipment on all sides. Round will not work for space and layout of the 5 pumps on the system. Containment and inspection are good points and have been worked in. Thx.