A Guide to Understanding P/V Valves
If you are totally clueless about pressure vacuum valves and you want to find out more, you have come to the right place! This concise guide will help you gain a better understanding of their operation, selection and installation. In short, pressure-vacuum (P/V) relief valves are safety devices that are intended to protect marine vessel systems such as cargo and storage tanks from imploding or exploding. This happens when the tank has not been properly vented even though there have been fluctuations in pressure due to changes in temperature and vapour quantity. The failure to vent fluid from an over-pressurised tank or taking in fluid to an under-pressurised tank will result in the tank potentially rupturing outwards or inwards respectively. Hence, it is essential to understand P/V valves and for you to properly size, select, install and maintain these valves.
There are two types of P/V valves – spring-loaded and weight operated. A conventional spring-loaded P/V valves consist of an inlet nozzle connected to the system, a movable disc that controls flow through the nozzle, a spring that controls the disc position. During normal operation, the disc remains in the closed position. When the pressure exceeds that of the set pressure, the spring will be pushed up and the valve will open, allowing fluid to flow out.
Figure 1: Spring-loaded P/V valve
For a weight operated valve, the control pressure is set by varying the weight on the lever. During normal operations, the opening force of the weight and the closing force of the piston balances each other out and is kept in a default open position. When the pressure falls, the control piston is pressurised with a pilot line that lifts the lever and moves the valve cone towards a close position, keeping the fluid in.
SELECTING A VALVE
There are several design factors for you to consider when selecting the right valve for your needs.
1. Easily interchangeable
The P/V valve chosen should be designed to be easily interchangeable in the event of any new upgrades. This will allow the marine vessel to comply with new or different safety standards in the future and from becoming obsolete. In this case, look out for any conversion kits or support from manufacturer for custom conversion solutions.
2. Simple design of valve
The P/V valve should be designed to fulfil its need yet be easy to maintain. One aspect to look out for is by having less moving parts that will also decrease the chances for mechanical failures. This reduces the need to lubricate and maintain these parts from time to time. For certain valves, some manufacturers provide their own maintenance kits which further simplifies the maintenance process.
3. Aerodynamic structure
P/V valves with better aerodynamic design does not impede airflow and enables gases to exit with high speed at low flow rates. This means that the vapours are emitted away at high speed, reducing the danger of any fire and the crew’s exposure to toxic fumes.
P/V valves should have resilient seals that are able to reseal the valves properly after each ventilation. This will prevent vapours from escaping from the gaps between the seals and hence the leakage of cargo. The low cargo emissions help to increase profits and lower the cost of operations, which could accumulate to a huge amount of revenue over time.
INSTALLING A VALVE
During installation, the P/V valves should be properly calibrated according to the requirements of the manufacturers, international standards and local regulations to ensure that the P/V valves functions as stated. The following tests can be conducted prior to installation by a third-party installation body.
1. Corrosion Test
According to BS ISO 15364:2016, the valve should be exposed to a 5% sodium chloride solution at 25 for a period of 240h and let dry for 48h. All movable parts should be in proper operation and corrosion deposits can be washed off.
2. Hydrostatic Pressure Test
According to BS ISO 15364:2016, the pressure retaining boundary of the device should be at least 150% of the maximum rated pressure without any rupturing, leakage of permanent distortion.
3. Performance Characteristics (Flow rates, operating sensitivity, flow resistance, velocity)
The performance characteristics stated by the manufacturers should always be verified by laboratory tests before installation to ensure it works as described.
We hope that this guide has provided you with greater insights on the working operations of the P/V valves, how to select and install them based on various criteria to make a more informed purchase.
This article is brought to you by Giantech Engineering Pte Ltd. Find out how Giantech may help meet your vessel’s pressure vacuum valves needs: