Danfoss PVG 32 Guía del usuario
Application Guide
PVG 32
Using flow or pressure control spools
powersolutions.danfoss.com
Revision history Table of revisions
Date Changed Rev
Feb 2017 First edition 0101
Application Guide
Using flow or pressure control spools
2 | ©Danfoss | Feb 2017 AB00000079en-US0101
References
Related literature: PVG 32 Proportional Valve Group Technical Information, 520L0344, BC00000038
Terms and definitions
Term Definition
CB Counterbalance
FC Flow Control
NO Natural Oscillation
PC Pressure Control
Pl Load pressure
PVG Proportional Valve Group
Ql Work Port Flow
Important user information
Danfoss is not responsible or liable for indirect or consequential damages resulting from the use or
application of this equipment.
The examples and diagrams in this manual are included for illustration purposes. Due to the many
variables and requirements associated with any particular installation, Danfoss cannot assume
responsibility or liability for the actual used bases on the examples and diagrams.
Reproduction of whole or part of the contents of this manual is prohibited.
The following notes are used to raise awareness of safety considerations.
W
Warning
Xxxxx
Identifies information about practices or circumstances that can cause a hazardous
situation, which may lead to personal injury or death, damage or economic loss.
C
Caution
Xxxx
Identifies information about practices or circumstances that can lead to personal injury
or death, property damage, or economic loss. Attentions help you identify a hazard,
avoid a hazard, and recognize the consequence
Note Identifies information that is critical for successful application and understanding of the
product.
Identifies a typical use of a functionality or parameter value. Use recommendations as a
starting point for the final configuration process of the system.
Application Guide
Using flow or pressure control spools
About this document
©Danfoss | Feb 2017 AB00000079en-US0101 | 3
Danfoss introduced the PVG 32 pressure control (PC) spools to the market in the 1990s to overcome
instability problems on applications and to control the opening of counterbalance (CB) valves. Typically, a
main boom on a crane could be applied with a CB valve for safety reasons. Inertia in the system could
lead to oscillations which will be damped when using PC spools.
In this document you can read about the hydraulic functionality and behavior of a PC spool compared to
our standard flow control (FC) spools. These hydraulic principles make it easier to know when to apply PC
spools instead of FC spools in different applications.
Application Guide
Using flow or pressure control spools
Application overview
4 | ©Danfoss | Feb 2017 AB00000079en-US0101
Normally, FC spools are used for most applications. The basic principle is shown in Basic principle of an FC
spool on page 5. The main spool works as a variable orifice controlling flow P>A/B and A/B>T,
simultaneously.
Basic principle of an FC spool
Pl is the hydraulic load pressure (determined by the size of the load) that acts in parallel with a spring on
one side of the compensator. The other side of the compensator is connected to the Pc pressure. The Pc
pressure will remain 7 bar higher than Pl. This also means that flow out of the valve is always directly
proportional to the opening area of the main spool. This can be seen in FC spool on page 5. A certain
position of the spool will always give the same amount of flow out of the valve independent of load
changes on the work ports.
FC spool
Application Guide
Using flow or pressure control spools
Functionality of a flow control spool
©Danfoss | Feb 2017 AB00000079en-US0101 | 5
As mentioned in the beginning of this document, instability is seen in systems that use FC spools in high
inertia systems. An example of such a system is shown in Natural oscillation example on page 6.
Natural oscillation example
APressure
BTime
Yellow line Pl
Red line Pc
At any given time, the compensator will try to keep the pressure drop across the main spool at the same
level and follow the pressure oscillations from the actuator attached to the valve. When the load pressure
is changing, the compensator will follow this frequency to keep the amount of flow from the valve at the
same level as the pressure.
So an FC spool offers no damping of the system but maintains a constant flow.
Application Guide
Using flow or pressure control spools
Flow control spools in oscillating systems
6 | ©Danfoss | Feb 2017 AB00000079en-US0101
A PC spool works as a variable orifice controlling flow P>A/B and A/B>T, like an FC spool, and also
controls LS>T. See the concept in Basic principle of a PC spool on page 7.
Basic principle of a PC spool
The Pc has a connection to the tank through a fixed orifice in series with a variable orifice (controlled by
spool displacement). This means that the LS signal is manipulated by the variable orifice. See the
characteristics of a PC spool in PC spool on page 7.
PC spool
Adrain variable connection from Pc to tank (manipulates the LS signal)
A(pc->Pl) physical opening area of the spool as a function of its position
Pl load pressure
Pc manipulated LS pressure + spring pressure
Ql flow out of the valve
The example in PC spool on page 7 shows a Ql curve, which is the flow out of the valve. Flow starts when a
pressure drop across the spool exist. So in this example, the flow will start and increase when Pc gets
higher than Pl. As a result, the spool deadband varies as the size of the load changes.
So for PC spools a certain position of the main spool is proportional to a certain Pc pressure.
Application Guide
Using flow or pressure control spools
Functionality of a pressure control spool
©Danfoss | Feb 2017 AB00000079en-US0101 | 7
One of the drawbacks of a PC spool is that flow out of the valve is dependent on the size of the load and
with variable deadband.
PC control to B port on page 8 illustrates a cut through PVB with PC control to one port.
PC control to B port
Application Guide
Using flow or pressure control spools
Functionality of a pressure control spool
8 | ©Danfoss | Feb 2017 AB00000079en-US0101
PC spools are beneficial when they are used with oscillating applications. The spool design applies
dampening to the port because flow from the valve varies with load change. See Natural oscillation
example on page 9.
Natural oscillation example
APressure
BTime
CFlow
Yellow line Pl
Red line Pc
Dotted line Ql
A certain position of the spool is given leading to a constant pressure, Pc. The load pressure, Pl is
oscillating. Over time, the pressure drop across the main spool (Pc minus Pl) varies, which leads to a
variable flow. The flow out, Ql, is also shown on the graph as a dotted line. The amount of dampening
applied is dependent on the relative difference in pressure between spool set point pressure and load
varying pressure. The relative difference is shown on the graph as ΔP1 and ΔP2.
Application Guide
Using flow or pressure control spools
Pressure control spools in oscillating systems
©Danfoss | Feb 2017 AB00000079en-US0101 | 9
The main purpose of a PC spool is to control CB valves. In general, the basic functionality of a CB valve is
to ensure non-moving actuator in case of hose break. See Lowering a load through a CB valve on page
10.
Lowering a load through a CB valve
APilot connections to force open CB
Lowering a load with a PC spool
A PC spool is more applicable when lowering loads in applications with CB valves.
A CB valve can be opened either by a pilot pressure or when the relief setting of the CB valve is reached
(shown as an adjustable spring in the previous figure). Normally, the relief setting and pilot pressure have
a ratio around 5:13. As a result, if the relief setting is 250 bar, the pilot pressure needed to open the valve
is a maximum of 50 bar.
In Lowering a load through a CB valve on page 10, a PC spool builds up pressure at the A-port. The
pressure on the A-port is used as pilot pressure to open the CB valve on the B-port. The load is held by the
CB valve and, with the pressure build-up on A-port, the opening of the CB valve is controlled. PC is
proportional to spool position and as a result the opening of the CB valve is proportional to the spool
position.
If the pressure is lowered with a FC spool, then there is no control of the pressure. So when the pressure
has increased to a level that opens the pilot for B-Port, flow is allowed back to tank. It also means that the
pressure in the A-Port will decrease with a risk of getting below the opening pressure for pilot control. As
a result, the system becomes unstable and tends to jump when lowering the pressure because the CB
valve switches between opened and closed.
Typically, a spool with FC in one direction (lifting) and PC in the other direction (lowering) is used for
applications with CB valves to provide the best controllability in both cases.
Application Guide
Using flow or pressure control spools
Pressure control spools with counterbalance valves
10 | ©Danfoss | Feb 2017 AB00000079en-US0101
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