FluidPower.Pro

Blogs

Blogs about hydraulic systems design.
Feel free to post your article here!

SUN cartridges and low temperatures

Recently we got a request from the customer to check the system components to work in a low-temperature environment up to -40°C. Most components in the customer’s system were SUN cartridges.

And I was really surprised by the mess of info on the Sun Hydraulic website about seals that SUN uses in their cartridges.

For example, on the “Seals: Materials of Construction” page you can find the manufacturer’s temperature range: from -22° to 230° F (-30° to 110° C) for Buna N (Nitrile) and -22 to 260° F (-30 to 125°C) for Polyurethane O-rings.

But, if you download Technical Product Information PDF from their library, you can find different temperature range for Buna N: -50°F to 200°F which means -45°C..+93°C and no info about Polyurethane O-rings.

This is why I was confused and had a lot of questions to SUN:

1. Why the different info is provided on SUN website?
2. Why there isn’t a choice for Polyurethane O-rings in seal material when selecting cartridges in the SUN catalog?
3. Why SUN has so high temperatures for the low environment for both Buna N and Polyurethane seals?

Really, all competitors have better low-temperature limits for their cartridges, for example:

– Parker’s Nitrile: from -40°C to +93.3°C
– Danfoss/Comatrol, Buna-N: from -40°C
– Bucher, Buna “N”: from -40°C to +120°C
– Doering, Buna N: from -42.7°C to +93.3°C
– Eaton, Buna N: from -40°C to +120°C
– Hydraforce’ Buna “N”: -40°C to +100°C; polyurethane: -54°C to +107°C.

Because it was critical for my application, I have sent a request directly to SUN to get answers to the questions above and have gotten next response: the correct low work temperature for Buna N is -30°C. Сoncerning the polyurethane seals – this is not a standard option, but any cartridge can be assembled with polyurethane o-rings that provides low temperature rate to -40°C. Not a big difference in the price, but because this is going to be a custom modification the delivery time for cartridges (statement on November 2021) is 15..16 weeks (what actually is not bad for SUN).

Just would like to share this experience with you…

Accumulators sizing for energy storage apps.

This week I worked on an accumulator sizing calculator for BOPs, so I have recovered my thermodynamic knowledge and just would like to note somewhere the materials summary and my thoughts about generic accumulator sizing for energy storage applications.

Let’s get generic formula

First of all, all accumulator calculations based on a generic formula of the polytropic process (Ideal Gas Law):

##pV^{n}=const \hspace{35pt} (1)##

Where the specific value of #n# corresponds to particular cases:
#n=0# for isobaric process
#n=+\infty# for isochoric process
#n=1# for isothermal process
#n=\gamma# for adiabatic process

Now, take a look at the image and apply this formula for the accumulator’s 3 major work stages:

Sizing Accumulators

Sizing Accumulators

Hydraulic motor case flushing

Hydraulic motor case flushing

Hydraulic motor case temperature

At the latest my project I used hydraulic motor Rexroth A2FLM710 (710cc). The motor works at 1400 rpm and provides 590 HP to the consumer. For safe motor work, I always try to keep the case temperature below 80*C. The easiest way to do this is a flushing flow adjustment.

In addition to the flow, you need to keep your eyes on the motor case pressure and try balancing to prevent overpressure in the motor case (check in the motor manufacturer’s catalog the max available case pressure to make longer life of motor shaft seals).

The values I have gotten: 21 GPM at 30 psi case pressure and in the worst-case scenario (max motor load, warm hydraulic oil) max case temperature was around 80*C

Hydraulic motor case flushing

Hydraulic motor case flushing flow and case pressure

There are no prescriptions or recommendations for valve or orifice size in motor catalogs for flushing flow, so the selection of flushing valves is a challenge.

Of course, you can find orifices (with different diameters) provided by the manufacturer with the motor in the motor’s catalog. But the flow and result case temperature will be different from application to application and the selection of the correct orifice is an engineering responsibility without any help or advice from the motor manufacturer.

Moreover, the manufacturer can’t provide all range of orifice diameters so the selection in the catalog is usually limited. And as you can see, sometimes values of flushing flow can be really huge and the only experience helps me to select the right flushing valve size at the beginning of the project.

I still believe, manufacturers can provide some diagrams/charts with correlation “power -> flushing flow” for approximate/preliminary estimation of the flushing valve size. Because I do not think everyone has a chance to make long tests during production and play with valves sizes…

What do you think?