Series, Parallel, and Tandem Spool Valve Circuits Explained

Two directional control valves can have the same number of spools, ports, and positions and still make a machine behave very differently. One valve may let an operator blend two functions. Another may stop every downstream function whenever a priority function is engaged.  

The difference often lies inside the valve bank: its series, parallel, or tandem circuitry.

Choosing among these arrangements affects more than hose routing. It influences operator control, actuator sequencing, available flow, pressure demand, heat generation, and the size of the pump and prime mover.  

What Does the Spool Do?

A spool is a closely fitted cylindrical component that moves inside the valve body. Raised areas called lands block passages, while grooves connect selected passages as the spool shifts. These connections direct pump flow to an actuator and route returning fluid toward the reservoir or another valve section.

In a sectional valve, each work section typically controls one cylinder, motor, or other function. Internal galleries connect those sections into a valve bank.

Manufacturers sometimes describe series, parallel, and tandem as spool options, section circuitry, or valve-bank circuitry. The implementation and ordering terminology vary, so the hydraulic schematic and manufacturer’s model code should always be treated as the final authority.

How a Parallel Spool Valve Circuit Works

In a parallel circuit, a common pressure gallery makes pump flow available to each work section. When one spool shifts, it connects that gallery to the corresponding actuator.

Because the sections share the available pump flow, an operator can often command more than one function at a time. This is one reason parallel circuitry is common on mobile equipment requiring flexible, overlapping movements.

Parallel does not mean that every active function receives equal flow. In a basic uncompensated valve, oil follows the path requiring the least pressure. If two spools are fully shifted, the lightly loaded actuator may move first while the more heavily loaded actuator slows or stops. The operator may be able to blend the functions by feathering the spools, but the result will still depend on load, spool opening, pressure loss, and available pump flow.

Pressure requirements do not normally add branch by branch in a parallel arrangement. At a given moment, the pump must develop enough pressure for the highest-pressure active load, plus circuit losses. However, the pump must provide enough total flow for the requested actuator speeds.

A parallel circuit is often a good starting point when:

• Functions must operate independently or in varying combinations.  

• The operator needs to blend movements.  

• There is no rigid priority between functions.  

• The circuit includes appropriate compensation when repeatable load-independent motion is required.  

For example, parallel circuitry may allow lift and tilt functions to be blended on material-handling equipment. Without pressure compensation, however, a large load difference can make one function dominate the available flow.

How a Tandem Spool Valve Circuit Works

A tandem circuit establishes a sequential flow path through the valve bank. Upstream sections have priority over downstream sections.

When the upstream spools are neutral, pump flow can continue through the open-center passage to the downstream sections. When an upstream spool is fully shifted, it diverts the available pump flow to that function and restricts or closes the path feeding the sections that follow it. As a result, a downstream function may slow or stop until the upstream control returns toward neutral.

Careful spool metering may leave some flow available downstream, but that behavior should not be mistaken for guaranteed flow sharing.

Tandem circuitry is useful when:

• One machine function must take priority.  

• Downstream interruption is acceptable or intentional.  

• The desired sequence can be established by valve-bank order.  

• A relatively straightforward open-center system is preferred.  

An OEM might use this arrangement when a steering, clamping, positioning, or other primary function must take precedence over an auxiliary movement. The tradeoff is reduced flexibility: valve-section order becomes part of the machine’s functional logic.

How a Series Spool Valve Circuit Works

In a series circuit, oil returning from an upstream actuator is routed back into the valve’s open-center passage instead of going directly to tank. That returning oil can then supply a downstream work section.

This makes it possible for more than one function to operate from the same stream of oil. Unlike an uncompensated parallel circuit, the downstream function is not simply competing for flow from a common pressure gallery. Its supply is the exhaust flow from the upstream function.

The pressure calculation is the critical limitation. Pump pressure may need to overcome the upstream load, the downstream load, and circuit losses. Those pressure requirements are therefore additive. The total must remain within the ratings of the pump, valve sections, actuators, lines, fittings, and relief system.

Downstream pressure also creates backpressure on the return side of the upstream actuator. That can change the upstream actuator’s available force and movement. With cylinders, the flow passed downstream may also differ from pump inlet flow because rod-side and cap-end displacement volumes are not equal.

Series circuitry can be appropriate when:

• Two functions are intended to operate together.  

• Reusing upstream return flow supports the desired motion.  

• The combined pressure requirement fits the system’s pressure budget.  

• Interaction between the upstream and downstream loads is understood.  

• Section order and actuator area ratios have been evaluated.  

Series circuits should be engineered from the complete operating cycle rather than selected as a general method for adding another function.

Do Not Confuse Tandem Circuitry with a Tandem-Center Spool

“Tandem” has a second common meaning in hydraulic valve terminology.

A tandem-center spool describes the connections inside a directional valve when the spool is in neutral. In the standard tandem-center condition:

• The pressure port is connected to the tank port.  

• Both work ports are blocked.  

This neutral arrangement can unload a fixed-displacement pump while keeping the actuator ports blocked. It does not describe priority among multiple valve-bank sections.

A tandem-center spool also should not be treated as a zero-leakage load-holding device. Spool valves require operating clearance and can leak internally. Suspended or safety-critical loads may require properly selected load-holding valves, such as pilot-operated checks or counterbalance valves, based on the application.

How to Select the Right Circuit

Start with the required machine behavior rather than a preferred valve part number.

• List the operating modes. Identify which functions operate alone, which must overlap, and which combinations must never occur.  

• Define priority. Determine whether any function must retain pump flow when another command is active.  

• Calculate simultaneous flow. Compare the combined actuator flow demand with pump delivery and acceptable function speeds.  

• Build the pressure budget. For a series circuit, include the combined load pressures, backpressure, and passage losses. For every circuit, verify all component and port ratings.  

• Match the pump architecture. Confirm whether the machine uses a fixed-displacement open-center system, a pressure-compensated closed-center system, or load-sensing control.  

• Specify neutral behavior separately. Work-section circuitry does not replace decisions about tandem-center, closed-center, motor, float, regenerative, or other spool conditions.  

• Evaluate metering and transitions. Consider what happens as each spool begins moving, reaches full stroke, and returns to neutral.  

• Verify the complete valve code. Flow rating and port size alone do not establish compatibility. Review the manufacturer’s schematic, section order, spool code, relief arrangement, power-beyond configuration, and allowable tank pressure.  

Common Selection Mistakes

A few assumptions frequently cause trouble:

• Treating parallel circuitry as an equal-flow divider  

• Assuming series operation provides additional pressure without a system-level penalty  

• Expecting downstream functions to remain available when a tandem priority section is fully operated  

• Confusing tandem valve-bank circuitry with a tandem-center neutral condition  

• Ignoring cylinder area ratios in a series circuit  

• Relying on a directional spool alone to hold a suspended load  

• Selecting a replacement valve solely by flow rating, port size, or number of spools  

Design the Valve Around the Machine

Parallel circuitry offers flexible access to pump flow, but uncompensated functions compete according to load and spool opening. Tandem circuitry creates clear upstream priority, but it can interrupt downstream movement. Series circuitry can support coordinated operation by passing return flow onward, but the pressure requirements and actuator interactions demand careful analysis.

No arrangement is universally best. The right choice is the one that produces the required motion throughout the complete operating cycle while staying within the system’s flow, pressure, thermal, and safety limits.

When a standard valve does not provide the needed combination of priority, metering, center conditions, and actuation, Bailey can help evaluate configurable or custom valve options for the application.

Safety note: Hydraulic systems may contain high pressure, stored energy, hot fluid, and loads capable of unexpected movement. Valve selection or modification should be based on the equipment documentation, manufacturer data, applicable standards, and qualified engineering review. Depressurize and secure equipment before service and never use a hand to search for a hydraulic leak.