Hydraulic design

One type of hydraulic system you don't want

One of our members (name withheld for reasons you'll understand in a moment) sent me this beauty:

"Our hydraulic power units are Kubota 86 hp and Deutz 76 hp 4 cylinder diesel systems. We use a 40 gpm gear pump with a splitter valve to create two circuits of 20 gpm, each with a variable flow control. We have a diverter valve upstream to combine flows if needed. The gear pump can handle the trash in offshore oilfield hydraulics and is inexpensive compared to piston pumps. Relief is set at 3000 psi.

A gear-type flow splitter would be better but is much more expensive. The flow controls we use are inexpensive - but the down side is they create unwanted heat. The flow thru relief valve is another problem source of heat - it dumps past the cartridge port back to tank during normal operation.

The biggest problem we experience is not enough cooling - trying to find a 12 VDC cooler that is not too expensive that can handle salt water (Gulf of Mexico oilfield operations). We looked at hydraulic fan motor but it was too expensive. This problem could make an interesting article for your newsletter."

An interesting article for this newsletter? As you can see, he did offer.

There are four problems here:

1. Being cheap to the penny;
2. Technical ignorance;
And as a result of 1 & 2:
3. Asking the wrong questions;
4. And looking for 'silver bullet'.

Let's take a look at each in more detail.

Being cheap to the penny

With hydraulics, like most other things, you get what you pay for. Design on price alone (especially if you don't know what you're doing) and you'll end up with a load of rubbish. Which is exactly what we've got here.

Oh sure, there's lots of hydraulic equipment users out there who don't know any better. And plenty of equipment builders who are short-sighted enough to accommodate them.

But if this guy can't build a properly engineered system, fit for the purpose and get his oilfield customers to pay that price - whatever it may be, he should go find something else to do. I mean, someone please remind me what the price of oil is at the moment? Sheesh.

Technical ignorance

If you are going to go 'cheap' you need to know what you're doing. By that I mean: what you can and can NOT get away with. I don't think it would be possible to come up with a more inefficient hydraulic system than the one we've got here. Let's take a quick look at what it comprises:

Gear pump - a.k.a. heat generating device
Spool-type flow divider - a.k.a. heat generating device
Variable flow control(s) - a.k.a. heat generating device
Pilot-operated relief valve - a.k.a. heat generating device

Have I upset anyone yet? Write this in blood: EVERY component in a hydraulic system is a heat generating device. But some are much worse, read: more inefficient, than others.

So if you grab a whole bunch of the most inefficient ones and throw them together, you better make sure you've got a BIG cooler. But wait, the 'budget' won't allow for a cooler will it? Darn it then, let's just cross our fingers and hope it will work OK without one. It's false hope.

Asking the wrong questions and looking for a 'silver bullet'

He knows he's got an overheating problem. And he's also at least vaguely aware that his choice of components is a contributing factor. But he's completely paralyzed by his wallet.

Asking: "How can I get this hydraulic system to run cool without sending any more money?" is futile.

Because it presupposes there is a magic solution. One which will defy the physical laws at the root of the problem. There is no such solution.

So what is the answer? Well despite this member's protestations to the contrary, I reckon I could design a power unit around a variable displacement piston pump and still be cost-competitive enough to consign his cheap 'n nasty version to the scrap heap. It would definitely be a lot simpler. And it would certainly work out cheaper in the long run.

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