In multi-actuator hydraulic systems, the order in which cylinders or motors operate is often just as important as the force or speed at which they move. A clamping jaw must close before a drill advances; a stabilizer must extend before a lifting arm rises. When this sequencing needs to happen reliably — without relying on electronic timers, sensors, or PLCs — a sequence valve is the component that makes it possible.
A sequence valve is a normally-closed, pressure-controlled valve that allows flow to a secondary circuit only after the primary circuit has reached a pre-set pressure. It introduces no moving parts to your control logic, no power supply dependencies, and no software. Sequencing is governed purely by hydraulic pressure.
This article focuses specifically on direct-acting sequence valves: what distinguishes them from pilot-operated designs, how to read and interpret their specifications, and how to select the right one for your system. For a detailed explanation of the underlying working principle, see our article How Does a Hydraulic Sequence Valve Work.
Quick Overview: Direct Acting Sequence Valve
What is a direct acting sequence valve and how does it work?
A direct acting sequence valve is a normally-closed, pressure-controlled valve where inlet pressure acts directly on a spring-loaded poppet or spool. When inlet pressure reaches the preset cracking pressure, the valve opens and allows flow to the secondary circuit — enforcing a strict operational order between two actuators without electronic timers, sensors, or PLCs. Sequencing is governed purely by hydraulic pressure.
Key specifications and what they mean (VS2C(OM) reference):
- Max flow – 30 L/min (G1/4) to 110 L/min (G3/4); undersizing starves the secondary actuator, oversizing wastes pressure budget
- Max PN – 350–400 bar depending on port size; must exceed peak system pressure including transient spikes, not just nominal operating pressure
- Pressure range – 10–180 bar (G1/4, G3/8) up to 50–400 bar (G3/4); cracking pressure must fall within this range or the valve cannot be correctly set
- Internal vs. external drain – use external drain when secondary circuit has significant back pressure; internal drain adds back pressure to spring force, raising effective cracking pressure above set point
- Contamination tolerance – ISO 4406 class 20/18/15 minimum; more tolerant than pilot-operated designs, suited to mobile equipment and harsh environments
Example
In an automated clamping and drilling machine, the VS2C(OM) sequence valve holds the secondary drilling circuit closed while the clamping cylinder extends and builds pressure. Once clamping pressure reaches the set point — confirming the workpiece is fully secured — the valve opens and the drill advances. No sensor or PLC is involved; pressure alone controls the timing.
Summary
Choose a direct acting sequence valve over a pilot-operated design when your application involves mobile equipment, cold climates, high contamination exposure, or simple two-circuit sequencing — and always verify cracking pressure, flow capacity, drain configuration, and the flow-pressure drop curve before finalizing selection.
Direct Acting vs. Pilot-Operated Sequence Valves
Sequence valves are broadly divided into two types based on how inlet pressure is converted into valve movement.
Direct-Acting Design
In a direct-acting sequence valve, inlet pressure acts directly on the valve element — typically a poppet or spool — against a calibrated spring. When inlet pressure rises to the spring’s set point (the cracking pressure), the valve opens and allows flow to the secondary circuit. The mechanism is straightforward and involves no intermediate pilot signal.
Pilot-Operated Design
In a pilot-operated sequence valve, a separate, smaller pilot signal is used to actuate the main valve element. This pilot can come from an external source or from the inlet itself (internal pilot). The amplification effect allows for greater precision and higher flow capacity, but introduces additional internal passages that require cleaner fluid and more careful design.
Comparison: Direct-Acting vs. Pilot-Operated Sequence Valves
| Feature | Direct Acting | Pilot-Operated |
| Response to pressure | Immediate — inlet pressure acts directly on valve element | Slight delay — pilot must build before main valve opens |
| Contamination tolerance | Higher — larger functional clearances | Lower — small pilot passages sensitive to particulate |
| Cold temperature performance | Better — fewer passages susceptible to viscosity increases | More sensitive — high viscosity can restrict pilot flow |
| Pressure accuracy | Good — suitable for most industrial applications | Higher — pilot amplification improves set-point precision |
| Mechanical complexity | Simple — fewer internal components | More complex — pilot mechanism adds parts |
| Typical flow capacity | Low to medium | Medium to high |
| Preferred environments | Mobile equipment, harsh conditions | Stationary industrial, precision applications |
As a general guide: if your application involves mobile equipment, cold climates, high contamination exposure, or relatively simple two-circuit sequencing, a direct-acting design is typically the more practical and durable choice.
Key Specifications and What They Mean
When reviewing a sequence valve datasheet, several parameters directly determine whether the valve is suitable for your system. The following definitions apply to direct-acting sequence valves as a class. Where specific values are cited, they are drawn from the VS2C(OM) datasheet — a direct-acting sequence valve available in four port sizes — used here as a concrete reference example.
Max Flow (L/min)
The maximum volumetric flow rate the valve is designed to pass. Exceeding this value causes excessive pressure drop across the valve and can result in heat generation, noise, and component wear. Undersizing the valve relative to circuit demand starves the secondary actuator.
The VS2C(OM) ranges from 30 L/min (G1/4 port) to 110 L/min (G3/4 port) depending on model size — a range that covers light industrial and mid-size mobile applications.
Max PN (bar)
The maximum allowable working pressure for the valve body and seals under continuous operation. This is not the same as the adjustable pressure range. The valve must be selected so that its Max PN comfortably exceeds the system’s peak operating pressure, including transient pressure spikes.
All four VS2C(OM) models are rated to at least 350 bar, with the G3/4 size rated to 400 bar.
Pressure Range (bar)
The band within which the cracking pressure can be adjusted. This defines the scope of the valve’s usefulness for a given application. If your required sequencing pressure falls outside this range, the valve cannot be correctly set regardless of adjustment.
The VS2C(OM) offers pressure ranges from 10–180 bar (G1/4 and G3/8) up to 50–400 bar (G3/4), allowing selection based on the specific pressure at which sequencing must trigger.
Setting Pressure (bar)
The factory reference pressure, or the recommended starting point for on-site adjustment. It is not a fixed value — it indicates the midpoint or nominal set point within the pressure range. Final setting should always be verified against actual system behaviour during commissioning.
Cracking Pressure (bar)
The minimum pressure differential at the valve inlet required to begin opening the valve. A lower cracking pressure means the valve responds more readily to small pressure rises; a higher cracking pressure provides a more definitive threshold before sequencing occurs. This value matters in systems where inlet pressure fluctuates under normal load cycling.
Across the VS2C(OM) range, cracking pressure varies from 1 bar (G3/4) to 7 bar (G3/8), reflecting the different spring characteristics in each size.
P/C Port Size (G thread)
The port thread standard determines physical compatibility with hydraulic lines and manifold connections. G (BSP) threads are common in European and Asian hydraulic systems. The VS2C(OM) is available in G1/4, G3/8, G1/2, and G3/4 — providing compatibility across a wide range of circuit sizes.
Mismatching port size to line size increases fluid velocity at the port and raises pressure drop, even if the valve’s flow rating is otherwise sufficient.
How to Select a Direct-Acting Sequence Valve for Your System
The following five-step framework is suitable for both engineers verifying technical fit and procurement managers translating system requirements into a sourcing specification.
Step 1 — Define Your Sequencing Pressure
Identify the pressure at which the primary circuit completes its function and flow should transfer to the secondary circuit. This becomes your target cracking/setting pressure. Confirm that it falls within the valve’s adjustable pressure range — not just below the Max PN.
Also determine whether back pressure in the secondary (return) circuit will affect valve accuracy. If return line pressure is significant, a valve with an external drain connection is preferred — internal drain designs allow back pressure to add to the spring force, raising the effective cracking pressure above the set point.
Step 2 — Match Flow Capacity
Determine the maximum flow rate that will pass through the sequence valve to the secondary circuit. Select a model whose Max Flow rating exceeds this value with a reasonable margin. Consult the valve’s flow-pressure drop curve to verify that the pressure drop at your operating flow is acceptable within your system’s pressure budget.
Step 3 — Confirm Pressure Ratings
Verify that the valve’s Max PN exceeds your system’s peak pressure — not just its nominal operating pressure. Hydraulic systems regularly generate brief pressure transients during load changes, valve switching, or pump start-up that can significantly exceed steady-state values. A common engineering practice is to ensure the valve’s Max PN is at least 1.25× the maximum anticipated system pressure.
Step 4 — Assess Environmental Conditions
Direct-acting sequence valves are generally more tolerant of challenging conditions than pilot-operated alternatives, but environment still affects selection:
- Temperature: Confirm the valve’s seal material rating covers your operating temperature range. Standard NBR seals are suitable for −20°C to +80°C; Viton seals extend this to approximately +200°C; HNBR seals provide flexibility down to −40°C.
- Contamination: Direct-acting valves typically require ISO 4406 cleanliness codes of 20/18/15 or better — coarser than pilot-operated designs (which need 18/16/13 or better). Confirm your filtration system meets the minimum requirement.
Vibration: Verify that the adjustment locking mechanism (lock nut, set screw) is appropriate for the vibration severity of the installation environment.
Step 5 — Verify Port Size and Mounting Configuration
Confirm the valve’s port thread standard (G/BSP vs. NPT) and size matches the hydraulic line and manifold specification. Inline mounting is common for direct-acting sequence valves; sub-plate and manifold mounting options may be available depending on the model.
The VS2C(OM) is offered in four port sizes (G1/4 through G3/4) to accommodate different circuit sizes within a single product family.
Reading a Flow-Pressure Drop Curve
Flow-pressure drop curves (ΔP vs. Q) are included in most hydraulic valve datasheets but are frequently overlooked during the selection process. Understanding what they show prevents a common specification error: selecting a valve that meets flow and pressure ratings on paper but introduces an unacceptable pressure loss in practice.
What the curve shows: For a given flow rate (Q, on the horizontal axis), the curve shows the pressure drop (ΔP, on the vertical axis) that occurs across the valve as fluid passes through it. This pressure drop is lost — it is unavailable to drive the secondary actuator.
How to use it during selection:
- Locate your expected operating flow rate on the horizontal axis.
- Read the corresponding ΔP value from the curve.
- Subtract this value from your available secondary circuit pressure to confirm the actuator still receives sufficient pressure to perform its function.
- If the pressure drop at your flow rate is too high, select a larger port size model with a higher flow rating.
The VS2C(OM) datasheet includes individual flow-pressure drop curves for each of its four port size variants, plotted at multiple pressure settings. Because the curve values in the product image are not fully legible at the available resolution, specific ΔP figures are not cited here — consult the full product datasheet for precise values.
Installation and Commissioning Considerations
Internal vs. External Drain
Direct-acting sequence valves can be configured with either an internal drain (pilot drain returns to the valve outlet/downstream port) or an external drain (pilot drain connects directly to tank).
When the secondary circuit has significant back pressure — common in systems with counterbalance valves, return line filters, or long return lines — an internal drain configuration causes that back pressure to add to the spring force. The result is that the valve opens at a higher pressure than the set point, causing sequencing to occur later than intended. In these applications, an external drain connection eliminates this interference and restores set-point accuracy.
Pressure Setting and Locking
Set the cracking pressure on-site by adjusting the spring preload (typically via a hex adjustment screw) while monitoring inlet pressure with a calibrated gauge. Always lock the adjustment after setting — using the lock nut or set screw provided — to prevent drift from vibration or accidental contact.
Begin commissioning at low flow and low pressure to verify sequencing behaviour before running the system at full load.
Filtration
At a minimum, ensure system fluid cleanliness meets ISO 4406 code 20/18/15 for direct-acting designs. Install filtration upstream of the valve if system cleanliness is uncertain. Regular fluid sampling and analysis is recommended, particularly in mobile equipment where ingression rates are higher.
A Note on Interchangeability and Sourcing
Buyers sourcing sequence valves as replacements for existing components — whether from European, Asian, or other manufacturers — should be aware that physical interchangeability is not the same as functional interchangeability.
Two valves may share the same port size and thread standard, yet differ in pressure range, cracking pressure, or flow capacity in ways that affect system behaviour. Always verify the replacement valve’s full specification against the original component’s datasheet before installation.
The VS2C(OM) designation indicates dimensional and specification compatibility with OM Group sequence valves of the equivalent series. Buyers familiar with OM-series components can use this as a reference point when evaluating the VS2C(OM) as a sourcing option. Verification against original OEM documentation is recommended as standard practice.
Direct-acting sequence valves offer a reliable, mechanically simple solution for pressure-triggered sequencing in hydraulic circuits. Their tolerance for contamination and cold temperatures makes them a practical choice for mobile and industrial equipment operating in real-world conditions.
Selecting the right sequence valve requires matching five key parameters to your system: sequencing pressure, flow capacity, maximum pressure rating, environmental conditions, and port configuration. Reading the flow-pressure drop curve is an often-skipped but important final step that confirms the valve will not rob the secondary circuit of the pressure it needs to function.
