Before you install a piping system for a new plant construction project or an existing plant expansion, it is important to conduct a hydrostatic test on the piping system. This test pressurizes the piping system and may expose instruments such as pressure indicators, switches and transmitters to pressures that exceed their capacity.
At Ashcroft – a leading authority in pressure and temperature instrumentation – we are often asked by customers what they can do to prevent their instruments from getting damaged in over-pressure applications. Read this article to learn about 5 common strategies that we recommend to keep your instruments protected during the testing process.
When you are done reading, you will be able to decide which approach is best for your specific needs. You will also be given additional resources that address other common questions we receive about keeping systems running safely and efficiently.
1. Isolation valves are the easiest way to protect instruments during hydrostatic testing.
Isolation valves play a crucial role in protecting instruments during hydrostatic testing. These valves come in various forms, including gauge valves, ball valves and needle-style valves.
- Gauge valves are designed to mount directly to the instrument to isolate it from the process.
- Ball valves feature a spherical design with a channel that can either allow the process fluid to flow through or isolate it, depending on the orientation of the channel.
- Needle valves have a pointed end similar in size to a pencil eraser, which gradually closes as the handle is rotated.
These options serve as manual methods for isolating instruments from the pressures experienced during hydrostatic testing procedures, ensuring the safety and integrity of the instruments involved.
Figure 1. Gauge valve example. 
2. Internal stops provide an added level of over-pressure protection.
When it comes to instruments, mechanical pressure gauges are often the most vulnerable to damage from overpressure. These gauges typically have a limit of 110 to 130% of their full scale before reaching the proof pressure. This is the maximum pressure they can handle without sustaining permanent damage. Exceeding 130% can cause the tube to stretch beyond repair.
Internal stops, or overload stops, are designed to restrain the pressure element motion by acting directly on the movement mechanism, as defined by ASME B40.100. They are often used to increase over-pressure protection by an additional 20%.
Figure 2. Internal stop example.
3. Pressure-limiting valves offer an effective solution for an automatic process.
Pressure-limiting valves (PLV) like the Ashcroft® PL02 streamline the manual process often associated with isolation valves during hydrostatic testing. In scenarios where testing is frequent or periodic, operators and installers would typically need to manually close each valve to protect the instrument every time. However, PLVs automatically isolate downstream based on a pre-set pressure threshold, offering a more efficient and automated approach. For instance, a pressure gauge positioned above a PLV set at 100 psi would be shielded, as the valve would automatically close at 100 psi to protect the instrument.
It's important to note that while a PLV is a mechanical device, there may be a slight delay between the set pressure and the actual valve closure, resulting in a hysteresis effect. To enhance protection further, incorporating a pressure snubber, such as the Ashcroft® PD02, above the PLV can help mitigate pressure spikes and protect sensor-based instruments.
Figure 3. Pressure limiting valve assembly.
4. A built-in over-pressure system means fewer potential leak paths.
Some mechanical pressure gauges, like the Ashcroft® T6500, are offered with an over-pressure system. The design is based on a traditional retard scale which allows for reading pressure with reasonable resolution for most of the dial arc. The remaining dial arc is used for over-pressure protection. Although this is only a solution for mechanical pressure gauges, it does offer some advantages over pressure-limiting valves. Fewer connections mean fewer potential leak paths. And if this is installed above a diaphragm seal it reduces the amount of system fill fluid allowing for a more responsive assembly. Most importantly you have some resolution in the over-pressure state whereas with a PLV the pointer would simply stop at the set point.
Figure 4. Ashcroft® T6500 Pressure Gauge with over-pressure system.
5. Remove the instrument from the process before testing.
The most effective way to safeguard instruments from overpressure is to completely remove them from the process. While this is straightforward for direct connections, it can pose challenges for connections involving an isolator.
For diaphragm seal assemblies threaded or flanged into a pipe tap, removal is usually uncomplicated. However, some seals may have an inline design with the lower housing installed in line with the pipe, making it difficult to move without disconnecting the top housing. In these cases, plugging the diaphragm seal's bottom housing with a blind top housing becomes essential. This not only aids in plugging the bottom housing during hydrostatic testing but also serves as a heat sink when welding the bottom housing to the pipe.
Figure 5. Ashcroft® blind top housing.
Isolation rings (otherwise known as Annular Seals) can also be troublesome during hydrostatic testing procedures because they are also very difficult to remove. It is important to have some form of instrument removal option on the isolation ring such as the Ashcroft® Safe Quick Release™ (SQR™). This allows the removal of the instruments without loss of fill fluid. This durable design is helpful both for the hydrostatic testing process as well as periodic maintenance and recalibration over time. 
Ready to learn more?
Now that you know five strategies for protecting your pressure instrumentation in preparation for hydrostatic testing, you are in a better position to determine which approach will work best for you. Here are a couple of related articles that may interest you.
- How Does Temperature Affect Pressure Gauge Performance?
- Do Measurement Instrument Accessories Affect Accuracy?
- Why Do I need a Steam Siphon and How Does it Work?
Or, to speak with someone directly, feel free to contact one of our product experts with any questions you have.
In the meantime, download our guide to learn more about pressure instrument assembly.
