When designing a high pressure pump the check valve design is usually overlooked and commercially available ones are used. However, the check valve design can dictate the motor size. The greater the back flow, the more the motor needs to compensate for the loss in flow. With the development of the P251K, our 1000 bar pump, we encountered some challenges that lead us to develop our own check valves. During the development we set ourselves several goals which included building a check valve with significantly reduced back flow when compared to the commercially available options. We aimed to reduce the required motor size and RPM to deliver 250g/min at 1000 bar of CO2. It’s important to understand how check valves operate. The key features of our check valves:
- Seal over a large pressure range, 0 – 1000 bar;
- Open and close quickly;
- Accommodate wide range of flows, 25g/min to 250g/min.
Control Valves and Check valves are tested against several reconised standards, API 598, ISO5208 and EN12266-1. For a check valve to pass one of these recognised standards, the leakage rate is defined ≤2 bubbles per 60 second period. Our aim was to achieve a rate lower than 1 bubble per 60 seconds. The most common design uses a ball and a seat. As the piston drives the flow towards the process the ball lifts with the pressure and closes due to the pressure differential in the process. A spring is normally added to ensure the ball securely seats and seals. As part of our design we explored a combination of materials using a silicone cardibde ball and stainless seat. The idea behind this was during use a slight deformation to the seat would occur when the ball impacts, creating a better sealing surface. The design went through a few iterations, with one of the biggest challenges reducing seal creep when the pressures exceeded 850 bar. PTFE comes in a variety of grades and modifiers to improve strength and lubricity. It therefore, can have a yield strength of somewhere between 1500 psi and 6000 psi. Once the pressure exceeds this value the material begins to flow into unsupported gaps within the valve. For a single use check valve this doesn’t cause a problem but re-using it becomes a challenge as the PTFE seal deforms and no-longer properly seals. Part of our design was to ensure the tolerances were good and that no gaps existed to allow creep of the PTFE. Finally, the check valve seat was polished to ensure that the check valve ball accurately created a seal. In conclusion, the accumulation of this knowledge, research and skills has enabled us to develop a highly efficient high-pressure CO2 pump capable of delivering CO2 at 1000 bar.