Low leak solenoid valves: why low leak rates are essential for your system
Leakage in solenoid valves is rarely immediately visible, but always costly. Whether it concerns energy loss in a compressed air network, measurement errors in an analytical instrument or test errors in leak testing equipment: even a minimal leak rate of a few cc per minute can make an entire system unreliable. This article explains how leakage occurs, what it costs and how Humphrey Products achieves demonstrably low leak rates.
⏱ approx. 8 minutes reading time
What is valve leakage and how does it occur?
Valve leakage is the unwanted flow of medium when a valve is closed or should be sealing. In an ideal valve, the flow in the closed state is exactly zero. In practice, virtually every valve lets through a certain amount of medium, ranging from negligible quantities to measurably significant flows that directly affect system performance.
Leakage in solenoid valves falls into two categories. Internal leakage occurs when medium leaks from the inlet side to the outlet side while the valve is closed, but the medium remains within the system. External leakage occurs when medium escapes from the system to the atmosphere via seals, threaded connections or a damaged housing.
Internal
Medium leaks from the high-pressure side to the low-pressure side when the valve is closed. Critical in pressure-holding systems and leak testing equipment.
External
Medium escapes to the environment via aged elastomers, cracks or loose connections. Leads to energy loss and pressure drop.
Continuous
A permanent leak flow, regardless of the switching status of the valve. Most costly due to continuous energy loss and compressor load.
Intermittent
Leakage occurs at specific moments in the cycle, such as during switching or during pressure build-up. Difficult to detect without logging.
Causes of leakage
The most common causes of unwanted leakage are aged, hardened or damaged elastomers that lose their sealing capability, corroded valve parts that no longer offer flat sealing surfaces, contaminants in the compressed air network that deposit on sealing surfaces, incorrect installation, and insufficient maintenance.
In addition to unwanted leakage, there are also leakage losses that are deliberately factored in by the manufacturer. In spool valves, a small amount of leakage along the spool is deliberately permitted as lubrication to reduce spool friction. This is a design choice with direct consequences for sealing accuracy.
The real costs of leakage
The direct and indirect costs of valve leakage are structurally underestimated. This is because leakage is rarely visible as a separate cost item: the compressor runs a little harder, maintenance is carried out a little more often, production stops a little more often. Each on its own seems minor. Together, they form a significant operational burden.
Energy loss
Compressors consume substantial electrical power. Leakage forces the compressor to work harder to maintain pressure. Every percentage of loss translates directly into higher energy costs. Studies estimate that 20 to 30% of the compressed air produced is lost through leakage in poorly maintained systems.
Maintenance costs
Leakage accelerates the wear of elastomers, filters and seals outside the valve because the increased compressor load puts more strain on the entire system. A higher wear frequency means shorter replacement intervals and higher maintenance costs.
Loss of production
Unplanned downtime due to leakage-related failures and repairs interrupts production. In automated systems, a leaking valve can bring an entire production line to a standstill until the cause has been found and remedied.
Measurement errors
In analytical instruments and leak testing equipment, a measurement error due to valve leakage is the most critical consequence. Even a leak of 0.001 PSI per minute can invalidate a test result if the specification is expressed as a pressure drop of 0.002 PSI over 30 seconds.
Calculating annual leakage costs
Annual costs = leak rate (CFM) × compressor power (kW/100 CFM) × operating hours/year × electricity costs (€/kWh)
Example: a leak of 1 CFM with a compressor of 20 kW/100 CFM, 6,000 operating hours per year and €0.25/kWh costs well over €300 per year per leak point. With dozens of leak points in a factory, this quickly adds up to many thousands of euros annually.
Hidden costs in analytical applications
In analytical instruments and leak testing equipment, the issue is not energy costs but measurement integrity. A valve with a leak of 0.05 cc/min in a system that must detect pressure drops of 0.001 PSI renders the instrument unusable. The costs are then not energy-related but quality-related: false test results, rejected products and loss of certification.
Poppet versus spool: the design difference that matters
The sealing principle of a valve largely determines the achievable leak rate. There are two fundamentally different approaches: the spool valve and the poppet valve. For applications in which low leak rates are essential, the distinction is decisive.
Spool valve
✗ Sealing via a close fit of the spool in the sleeve: some clearance is always needed
✗ A small amount of leakage along the spool is deliberately designed in as lubrication
✗ Leak rate increases with wear and contamination of the sleeve
✗ Sensitive to contamination in the medium
✓ High flow capacity for the same size
✓ Low switching power at larger sizes
Poppet valve
✓ Sealing via an elastomer poppet on a flat seat: bubble-tight possible
✓ No deliberate leak lubrication needed: sealing is the primary mechanism
✓ Leak rate remains consistent over millions of cycles
✓ Tolerant of light contamination: the poppet cleans the seat with every stroke
✗ Slightly higher switching power for larger sizes
✓ Suitable for vacuum, high pressure and aggressive media
The poppet principle achieves sealing via an elastomer poppet that presses forcefully onto a flat metal or plastic seat with every closure. The contact line is a circle and the contact pressure is high and uniform. This makes true zero leakage theoretically achievable and, in practice, extremely low leak rates structurally demonstrable over a long service life.
Balanced poppet: the Humphrey advantage
Humphrey's balanced poppet design also eliminates the pressure dependence of the seal. With an unbalanced poppet, the system pressure works with or against the closure, making the leak rate pressure-dependent. With a balanced poppet, the net force of the system pressure on the poppet is virtually zero, so that the sealing force is determined solely by the spring and the solenoid. This delivers a consistent, pressure-insensitive low leak rate over the full working pressure range.
How Humphrey Products achieves low leak rates
Humphrey Products has been developing and manufacturing solenoid valves for decades and has more than 30 years of specific experience with ultra-low leakage applications for analytical instruments, medical equipment and leak testing equipment. The following design principles underlie the proven low leak rates.
Poppet design as the starting point
The entire Humphrey poppet valve family is based on the direct sealing principle in which an elastomer poppet seals directly on a seat. There is no spool, no fit and no deliberate leak clearance. The HK5 valve and the broader MiniMizer series have been specifically developed for applications in which the leak rate must be as close to zero as possible.
Material combinations for long-lasting sealing
Elastomer ageing is the most common cause of increasing leakage over the lifetime of a valve. Humphrey selects elastomer compounds based on the specific application: Buna nitrile for standard air applications, FKM (Viton) for aggressive media and higher temperatures, and EPDM for aqueous media. The valve series are designed for a minimum service life of 50 million cycles while maintaining the specified leak rate.
Product certification per assembly
For critical applications in analytical instruments, Humphrey certifies every individual assembly before shipment. This means that not a sample from a batch is tested, but each product individually. The certification uses the customer's own test methodology, so that the guarantee aligns directly with the application specification rather than a generic manufacturer's standard.
Minimal dead volume in manifolds
In analytical instruments, not only the leak rate of the valve itself is critical, but also the dead volume in the liquid or gas path. Humphrey's manifold designs for low leak applications minimise the non-functional internal volume by actively optimising the geometry of the manifold block for the fluid architecture. Less dead volume means a faster pressure response, less influence of leakage on the total system volume and higher measurement accuracy.
| Specification | Humphrey performance | Application |
|---|---|---|
| Standard leak rate | ≤ 0.05 cc/min at 250 PSI (17.2 bar) | Leak testing equipment, proven over millions of cycles |
| Ultra-low manifold | ≤ 1 PSI pressure drop at 100 PSI over 5 minutes | Compact analytical instruments (HK5-based) |
| High-performance leak test | ≤ 0.002 PSI pressure drop over 30 seconds at 100 PSI (≈ 0.0025 scc/min) | Precision leak testers, 20× better than the customer specification |
| Service life guarantee | Specified leak rate maintained over ≥ 50 million cycles | All poppet series; MiniMizer, HK5, balanced series |
| Cycle test interval | Tested every 50,000 cycles over 200,000 total cycles | High-performance leak test manifold validation |
Cases: ultra-low leakage in practice
Case 1: Ultra-low leakage manifold for a compact analytical instrument
A manufacturer of laboratory analysers developed a new instrument with a single-stroke pump as the compressed air source, eliminating the need for an external compressor. This placed extremely high demands on valve leakage: any measurable leak would immediately negate the pressure build-up of the pump and render the instrument unusable.
The valves the customer had initially selected turned out to have an unacceptable leak rate. Humphrey's Engineered Solutions team introduced the HK5 valve, modified for virtually zero leakage, and designed a cube-shaped manifold instead of a linear layout. This cube shape made it possible to mill internal connecting channels that minimised the dead volume while the valve fitted within the original space envelope of the instrument.
Each assembly was certified to a maximum pressure drop of 1 PSI at 100 PSI over five minutes, using the customer's own precision test methodology. Humphrey set up a special measuring set-up in the factory for this purpose in order to measure and document the extremely low leak rates.
Case 2: High-performance manifold for precision leak testing equipment
A manufacturer of leak testing equipment built systems that had to be able to detect pressure differences of 0.00001 PSI and calculate the corresponding leak rate. The valves in the test system itself therefore had to have a lower leak rate than the product being tested, otherwise the measurement would be invalidated by the test set-up's own leakage.
Humphrey developed a manifold with MiniMizer valves that exceeded the customer specification by a factor of twenty. Validated assemblies were subjected to 200,000 switching cycles, with a precision leak test performed every 50,000 cycles. At each of the four test points, the pressure drop due to leakage remained below 0.002 PSI over 30 seconds at 100 PSI, corresponding to a leak rate of approx. 0.0025 scc/min.
This level of performance enables the customer to carry out leak tests on automotive systems, medical equipment, electronic enclosures, food packaging and aerospace components, with the valves in the test set-up making no measurable contribution to the test result.
Detecting and measuring leakage
For engineers who want to track down leakage in existing systems or validate new systems, various methods are available, each with its own range of application and level of accuracy.
Auditory detection
Listening for a hissing sound is the simplest method. Effective for large leaks in quiet environments, but not sensitive enough for small leaks or in noisy industrial environments.
Soapy water test
Applying soapy water to connections and fittings makes bubble formation visible in the case of external leakage. Simple and effective for localisation, not suitable for quantification or internal leakage.
Pressure decay measurement
The system is pressurised, shut off and the pressure drop over time is measured. Accurate and quantifiable. The standard method for validating valves in leak testing equipment. Can detect leak rates of 0.001 PSI/min with suitable transducers.
Differential pressure test
Pressure is applied simultaneously to the product under test and a known reference volume. The system compares the pressures and detects any difference as leakage. Higher sensitivity than single pressure decay.
Vacuum decay measurement
Vacuum is applied to the part under test, after which the valve closes. A rise in pressure indicates leakage from the outside in. Effective for closed components and medical equipment.
Ultrasonic leak detection
Ultrasonic detectors analyse high-frequency sound signals from compressed air leaks. Effective in noisy environments where auditory detection fails. Suitable for periodic inspection rounds in production environments.
Leak rate expressed as pressure drop or volumetric flow
Engineers use two units for leak rates: volumetric (cc/min or scc/min) and as pressure drop per unit of time (PSI/min or bar/min). The conversion between the two depends on the system volume. Humphrey specifies both: the standard poppet series has a specified leak rate of ≤ 0.05 cc/min at 250 PSI, while high-performance manifolds are certified as a maximum pressure drop per unit of time because that corresponds directly to how leak testing equipment expresses its specifications.
When is a low leak solenoid valve necessary?
Not every application requires an ultra-low leak solenoid valve. In standard pneumatic actuation applications in which valves control cylinders or actuators, a certain degree of leakage is acceptable as long as the system pressure is maintained. But there are fields of application in which a low leak rate is a hard technical requirement rather than something desirable.
Leak testing equipment is the most demanding category: the valves in the test set-up must have a lower leak rate than the product being tested, otherwise the measurement is invalid. Automotive systems, medical equipment, electronics, food packaging, HVAC and aerospace components are all tested with this type of equipment.
Analytical instruments such as gas chromatographs, mass spectrometers, elemental analysers and nitrogen or protein analysers depend on precisely controlled gas flows. Any leakage in the fluidic system disrupts the calibration and makes quantitative results unreliable.
Medical equipment such as ventilation systems, dialysis equipment and surgical instruments places demands on valve leakage for safety and regulatory reasons. An uncontrolled leak of liquid or gas in a medical instrument is a risk to the patient.
Energy-conscious industrial systems with large numbers of valves benefit from low leak rates through a demonstrable reduction in compressor energy consumption, longer maintenance intervals and less unplanned downtime.
Checklist for engineers: when do you specify a low leak valve?
Use a low leak solenoid valve when the system holds a closed volume that must not drain (leak testing equipment, analytical instruments), when the pressure is supplied by a pump without a reservoir, when leakage directly affects measurement results, when the medium is scarce or expensive, or when regulations require a demonstrably low leak rate (medical, food-grade, potentially explosive environments).
Humphrey low leak solenoid valves at Inacom
Inacom is the official distributor of Humphrey Products in the Benelux. The range includes the balanced poppet series, the MiniMizer series for ultra-low leakage applications, the iDP inert diaphragm poppet valves for aggressive media and custom manifold assemblies. Humphrey's Engineered Solutions programme is open to applications that go beyond standard products. Contact Inacom for application advice and product specifications.
