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.

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.

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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