Recently I was involved in test of the suction pressure of pump, that faulty because cavitations. It was an interesting experience and I noted that not everyone understand requirements for pump’s inlet pressure from manufacturers. I made a diagram above to add more clarity for everyone and added below my answers for most popular questions about vacuum pressure.
Air in oil is not good, if there’s a lot of air in the oil, the hydraulics don’t behave the way you expect them to:
– efficiency losses
– cavitation
– loud noise
– faster oil ageing
– higher temperatures
– changes in oil properties
– control problems
Air in oil impacts the life of the oil and the life of the components in the system. The components can be damaged because of the air in the system. The cavitation effects cause damage to components and create loud noise, micro-diesel effects and increases in temperature make oil age more quickly. This causes deposits to alter the oil viscosity and reduces the fluid thermal conductivity. Free air in the oil also increases the compressibility of oil and makes control unresponsive and imprecise leading to certain safety problems.
How does the air get into the oil?
There are a lot of ways, but the major ones are:
– by liquid sloshing in the tank
– by the pump suction the air
– through damaged seals in cylinders
– as a result of flawed filter tank design
– during maintenance works
– when equipment is connected
So what are the ways to decrease air bubbles in the hyd tank?
The company where I’m currently working manufactures and ships equipment around the world.
Although the SI unit for pressure is the Pascal (Pa), different countries use their own kind of “standards” for pressure measuring, and when designing the system I need to keep in mind what country our equipment is shipping to. Below are just my notes/recommendations, which I personally use in my practice in case if customer doesn’t have specific requirements for pressure gauges (brand/units). Since our equipment is designed and manufactured in Alberta and Texas, I use a primary scale that matches the region and “psi” scale as a secondary one.
| Region | Unit | Gauge unit(s) scale | Example |
|---|---|---|---|
| North America South America |
psi (Pounds per Square Inch) |
psi |
 |
| Australia China |
kPa or MPa (KiloPascal or MegaPascal) |
Dual: psi/kPa |
 |
| Western Europe Arabic Countries |
bar (Bar) |
Dual: psi/bar |
 |
| India South Korea South Asia |
kg/cm² (Kilogram per square centimetres) |
Dual: psi/kg/cm² |
 |
The unit converter you can find by this link.
Most popular pressure gauge scale ranges:
30 psi = 207 kPa = 0.207 MPa = 2.068 bar
60 psi = 413 kPa = 0.414 MPa = 4.14 bar
160 psi = 1,103 kPa = 1.1 MPa = 11.03 bar
200 psi = 1,379 kPa = 1.38 MPa = 13.79 bar
600 psi = 4,136 kPa = 4.1 MPa = 41.4 bar
1,000 psi = 6,895 kPa = 6.9 MPa = 69 bar
2,000 psi = 13,789 kPa = 13.8 MPa = 138 bar
3,000 psi = 20,684 kPa = 20.7 MPa = 207 bar
5,000 psi = 34,474 kPa = 34.5 MPa = 345 bar
Hydraulic oil Density is the ratio of its mass to the volume of space it occupies:
## \rho=\frac{m}{V} ##
The accepted units of measurement for density according to ASTM are kilograms per cubic meter (kg/m3, SI unit) or grams per milliliter (g/mL).
Hydraulic oil Specific Gravity (or “Relative Density”) is the ratio of Hydraulic oil density to water density at the specific temperature:
## SG_{oil}=\frac{\rho_{oil}}{\rho_{water}} ##
Substance with SG <1 will float on water (like hydraulic oil); substance with SG >1 will sink in water (like honey).
Next, both hydraulic oil density and Specific Gravity vary with temperature and pressure.
Reference: [https://www.internetchemistry.com/chemical-data/water-density-table.php]
Reference: [https://webbook.nist.gov/chemistry/fluid/]
This is why the ASTM D1298-12b “Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method” states that accurate determination of the API gravity, density or relative density (specific gravity) uses a standard temperature of 60 degrees F (15 degrees C).
Reference: [https://www.astm.org/d1298-12b.html]
You can find in the oil specification table of every hydraulic oil manufacture either Oil Specific Gravity or Oil Density at the specific temperature of 60°F or 15°C.
As a result, to calculate Specific Gravity or Hydraulic oil density at 15°C and, respectively, Hydraulic oil density or Specific Gravity at 15°C. By ASTM D1298-12b water density at 15°C is 0.999103 g/ml, therefore:
## SG_{oil}=\frac{\rho_{oil}}{0.999103} ##
## \rho_{oil}=\frac{SG_{oil}}{0.999103} ##