June_AMP_Digital

ADVANCED MATERIALS & PROCESSES •

JUNE 2014

HTPRO

This is why it is necessary to size the ro-

tameter for each application. When

sized correctly, flow rate can be deter-

mined by matching the float position to

a calibrated scale on the outside of the

instrument. Many rotameters come

with a built-in valve to manually adjust

flow. Several shapes of floats are avail-

able for various applications.

Both glass and metal rotameters are

available. Glass and plastic rotameters

cost less and are more accurate than

metal tapered-tube rotameters (Fig. 3b),

but may not be able to provide the dura-

bility and reliability needed in a manufac-

turing environment. Metal rotameters

are reliable, but the machined tapered

tube limits the flow measurement range

(turndown). Another limitation is that

metal rotameters typically have brass or

aluminum bodies, which can make them

unsuitable for use in certain gases (am-

monia, for example).

Slotted-cylinder types

The flowmeter most commonly used in

the process industries substitutes a slot-

ted cylinder for the tapered tube (Fig.

3c). Compared with a metal rotameter,

a greater selection of construction ma-

terials and a flow turndown of at least

25:1 (vs. 3.6:1) are provided.

The lower portion of the float is a piston

that can “plug” the slot in the cylinder

wall. The float rises until enough of the

slot has opened to create equilibrium

between the two upward-acting flow

forces and the single downward-acting

force. As for rotameters, when in this

equilibrium position, float height is pro-

portional to flow rate. The basic equa-

tions for tapered tube and slotted

cylinder flowmeters are similar, with

their flowmeter coefficients (K factors)

accounting for any differences.

Flowmeter accessories

Regardless of the design of variable-area

flowmeters, flow measurement is taken

at some equilibrium point where the

fluid flow force is balanced by an oppos-

ing force exerted by a “flow element”

(such as a float). Either the force of grav-

ity or a spring is used to return the flow

element to its resting position when the

flow lessens. Gravity-operated flowme-

ters (Fig. 3a–c) must be installed in a

vertical position, while vane or spring-

operated devices (Fig. 3d–f ) can be

mounted in any position.

Some variable-area flowmeters can be

provided with position sensors and

transmitters (pneumatic, electronic, dig-

ital, and fiber optic) for connecting to re-

mote displays or controls. Most flowme-

ters have only flow alarm output signals,

although some provide a continuous sig-

nal that represents the flow rate.

A variable-area flowmeter or rotameter

is typically provided with calibration

data and a direct-reading scale for air or

water (or both). To size a meter for other

service, the actual flow must be con-

verted to a standard flow. Instrument

manufacturers use different standard

flow units. For liquids, the standard flow

is the water equivalent in gal/min at 70ºF

and 10 psi (20ºC, 69 kPa); for gases, it is

the air equivalent in standard cubic feet

per minute (scfm) at 70ºF and atmos-

pheric pressure. Tables listing standard

water and/or air equivalent values are

available from flowmeter manufactur-

ers, who also might provide slide rules,

nomographs, and computer software for

flowmeter sizing.

HTPRO

Look for Part 2 of this article in the Sep-

tember 2014 issue of HTPro covering se-

lection basics, sizing, mass flowmeter

overview, and FAQs about flowmeters.

For more information:

Daniel H. Herring

(The Heat Treat Doctor) is president,

The Herring Group Inc., P.O. Box 884,

Elmhurst, IL 60126-0884, 630.834.3017,

dhe r r i ng@he at- t re at- do c to r. com, heat-treat-doctor.com

Fig. 3 —

Variable-area flowmeters: (a) glass/plastic tapered tube rotameter, (b) metal tapered tube rotameter, (c) slotted metal cylinder,

(d) vane type, (e) piston meter with spring-loaded orifice piston over a tapered plug, and (f) tapered tube with spring.

Flow

D Tube

D Float

Flow

(a)

Flow

D Tube

D Float

Flow

Indicator

(b)

Indicator

Slot

g P

Open

slot

length Flow

Flow

(c)

Vane P

Flow Flow

(d)

Moving orifice

Metering cone

F spring

Flow Flow

(e)

F spring

Flow Flow

(f)