# Special fluid flow rate control problem

B

#### Brian Howerton

I'm developing a test to look at the stress-strain characteristics of polyurethane foams under hydrostatic loading. The test requires a constant strain rate (e.g. constant flow rate into the test chamber) and readings of chamber pressure (stress) and the amount of fluid pumped into the chamber (strain) as a function of time. I can measure the pressure, but have not figured out a good way to control the flow rate or measure it. We need a scheme to set up a constant flow rate into th pressure vessel where the amount of fluid is small (8 in^3) but the rate is high (up to 40 gal/sec) and the duration is short (0.07 sec). Accuracy on the integrated volume measurement needs to be better than 3 cm^3. We can use water or hydraulic oil (water preferred) and have to reach chamber pressures near 2000 psi. Any ideas?

D

#### david mertens

I would suggest using a mechanical injection system (e.g. syringe, tube with piston) actuated automatically with a controlled speed. The accuracy of the flow would then only depend on the speed of the actuator and that should be
easily controlled with a hydraulic actuator. A spring loaded actuator could also work if the flow does not have to be controlled while injecting but only set to a fixed value before the experiment.

B

#### Bob Peterson

Premeasure the material you want to inject into a syringe.

Depress the syringe in 0.07 sec (using a hydraulic servo cylinder maybe).

The controls should be relatively straightforward.

Bob Peterson

T

#### T. Connolly

We do something very similar at a pressure of 2.1 Kilobars but at significantly higher volumes. Here goes, I hope this crude text sketch comes across ok.<br>
<pre>

Supply
|
| check valve(4)
|-o>-----------|---------------<o-|
| |
| drive section |
| |-------------| |
|-----------| || |-----------|
| |---------------||-------| |lf. pump
|-----------| || |-----------|section
|rt.pump |-------------| |
|section |
| |
|-<o--------------|------------o>-|
|
|
discharge
</pre>
What you are looking at is a reciprocating fixed displacement hydraulic operated pump. Also called an intensifier. It uses a low pressure
hydraulic fluid (or pneumatic) to drive the center section at a fixed speed (called the drive section). Attached to each end of the drive
section are pumping sections, or smaller cylinders with a piston attached to the drive. As the drive section shifts to the right in the
diagram above, fluid is dischrged out the right pump section and fluid is drawn into the left pump section like a syringe through the check
valves. At the end of the stroke, the unit has discharced an exact volume of fluid determined by the size of the pump section cylinder. The left pump section is fully charged for the next pump cycle. In our case we use hydraulic oil in the drive section and deionized water is the pumped fluid. You can use any fluid mix you like, including air/gas if you design it properly. You may have to construct this device yourself or have a shop that specializes in hydraulic intensifiers build a special for you. Controlling the speed is fairly simple, and can be done easily enough with meter in/out mechanical flow controls on the drive section, skipping electronic controls altogether.

T

#### T. Connolly

We do something very similar at a pressure of 2.1 Kilobars but at significantly higher volumes. Here goes, I hope this crude text sketch comes across ok.<br>
<pre>

Supply
|
| check valve(4)
|-o>-----------|---------------<o-|
| |
| drive section |
| |-------------| |
|-----------| || |-----------|
| |---------------||-------| |lf. pump
|-----------| || |-----------|section
|rt.pump |-------------| |
|section |
| |
|-<o--------------|------------o>-|
|
|
discharge
</pre>
What you are looking at is a reciprocating fixed displacement hydraulic operated pump. Also called an intensifier. It uses a low pressure
hydraulic fluid (or pneumatic) to drive the center section at a fixed speed (called the drive section). Attached to each end of the drive
section are pumping sections, or smaller cylinders with a piston attached to the drive. As the drive section shifts to the right in the
diagram above, fluid is dischrged out the right pump section and fluid is drawn into the left pump section like a syringe through the check
valves. At the end of the stroke, the unit has discharced an exact volume of fluid determined by the size of the pump section cylinder. The left pump section is fully charged for the next pump cycle. In our case we use hydraulic oil in the drive section and deionized water is the pumped fluid. You can use any fluid mix you like, including air/gas if you design it properly. You may have to construct this device yourself or have a shop that specializes in hydraulic intensifiers build a special for you. Controlling the speed is fairly simple, and can be done easily enough with meter in/out mechanical flow controls on the drive section, skipping electronic controls altogether.

T

#### T. Connolly

We do something very similar at a pressure of 2.1 Kilobars but at significantly higher volumes. Here goes, I hope this crude text sketch comes across ok.<br>
<pre>

Supply
|
| check valve(4)
|-o>-----------|---------------<o-|
| |
| drive section |
| |-------------| |
|-----------| || |-----------|
| |---------------||-------| |lf. pump
|-----------| || |-----------|section
|rt.pump |-------------| |
|section |
| |
|-<o--------------|------------o>-|
|
|
discharge
</pre>
What you are looking at is a reciprocating fixed displacement hydraulic operated pump. Also called an intensifier. It uses a low pressure
hydraulic fluid (or pneumatic) to drive the center section at a fixed speed (called the drive section). Attached to each end of the drive
section are pumping sections, or smaller cylinders with a piston attached to the drive. As the drive section shifts to the right in the
diagram above, fluid is dischrged out the right pump section and fluid is drawn into the left pump section like a syringe through the check
valves. At the end of the stroke, the unit has discharced an exact volume of fluid determined by the size of the pump section cylinder. The left pump section is fully charged for the next pump cycle. In our case we use hydraulic oil in the drive section and deionized water is the pumped fluid. You can use any fluid mix you like, including air/gas if you design it properly. You may have to construct this device yourself or have a shop that specializes in hydraulic intensifiers build a special for you. Controlling the speed is fairly simple, and can be done easily enough with meter in/out mechanical flow controls on the drive section, skipping electronic controls altogether.