Metso NB723, Installation, Maintenance And Operation Instructions

7 NP 72 en

3

1

INTRODUCTION

1.1

General

The NP700 positioner is used for positioning double or sin-
gle acting actuator.

1.2

Principle of operation

Operation is based on the force balance principle. One force
is caused by the signal pressure (IN) and acts on the dia-
phragm (8), and the other is the compression force caused
by the feedback spring (60.1). The latter is directly propor-
tional to the position of the lower end of the spring, which
is in turn dependent solely on the position of the actuator
shaft via the lever (33), the cam (29), the positioner shaft
(26) and the coupling (52).

When the equilibrium between the forces mentioned
above is disrupted, the beam (5) moves in the direction of
the stronger force. The end of the beam moves the spool
(44.2) in the bore of the body (44.1). When diverted from
the equilibrium position, the spool guides the supply air to
one side of the actuator piston and the air on the other side
of the piston outside.

When the spool is in the equilibrium position the small vol-
ume of leakage past the spool creates an equal pressure on
either side of the unloaded cylinder piston. This pressure is
roughly 0,7 x the supply pressure.

If the signal pressure is altered, the spool moven out of the
equilibrium position and causes a pressure difference in the
cylinder. The piston moves in the direction of the difference
pressure until the compression force of the feedback spring
generated by the change in position is in equilibrium with
the force generated by the signal pressure.

Thus each signal pressure value corresponds to a single
actuator position. If an external force acts on the actuator
shaft, the shaft tends to move in the direction of the torque.
This alters the position of the spool via the feedback system
so that the pressure difference arising in the cylinder offsets
the effect of the external torque.  

Parts list for Figure 1:

Item

Part

5

Beam

8

Diaphragm  piston

26

Shaft

29

Cam plate

33

Lever

44.1

Pilot valve body

44.2

Pilot valve spool

46

Changeover piece

52

Coupling

60.1

Feed-back spring

60.3

Range adjustment

67

Zero adjustment

1.3

Marking of positioner

The positioner has an adhesive ID plate, see Fig. 2.

The ID plate contains the following information (from top to
bottom):



Full type designation of the positioner



Input signal



Max. supply presssure



Ambient temperature range



Manufacturing series number

An additional plate, Fig. 3, has markings:



Filter regulator (-K)



Temperature range 

Fig. 1

Schematic diagram

Fig. 2

ID plate

Fig. 3

Additional plate