Handbook of Water and Wastewater Treatment Plant Operations - Chapter 9 ppt

While it is a useful way to obtain a ballpark estimate for the flow rate as part of a preliminary field study, this technique is not suitable for routine measurements required in water a

Flow Measurement

Water is the best of all things.

Pindar (C 522–C 438 B.C.E) Olympian Odes

Flow is measured…one cannot afford not to mea-sure…because the competitive environment dictates that one does “not” have the luxury of producing “wastefully.”

Someone is always watching; if not the plant manager, then the vice-president of finance; if not the plant neigh-bors, then the EPA; and, of course, there are laws of physics to keep everyone honest 1

9.1 INTRODUCTION

Flow is one of the most difficult variables to measure

accurately If we wanted to use an approximate (but very

simple) method to determine open channel discharge, we

would measure the velocity of a floating object moving

in a straight uniform reach of the channel If we know the

cross-sectional geometry of the channel and the depth of

flow is determined, we can compute the flow area From

the relationship Q = A ¥ V, we can estimate the discharge Q

The average velocity of flow in a reach is approximated

by timing the passage of the floating object along a

mea-sured length of channel

While it is a useful way to obtain a ballpark estimate for the flow rate as part of a preliminary field study, this

technique is not suitable for routine measurements required

in water and wastewater treatment plant operations

Another simple (but impractical) method of determining flow is the weight per unit time method This method

assumes a basic premise of fluid mechanics: mass is a

con-served quantity Simply, the mass entering a system is equal

to the mass leaving the system when both are measured over

the same time interval Using the weight per unit time

method to measure flow requires catching the flow in a

container and weighing it over a given interval of time The

impracticality of using this method in water and wastewater

operations can be seen in closed-loop processes commonly

associated with chemical applications Consequently, other

methods must be used to obtain flow measurements

Any seasoned water and wastewater operator knows that flow measurement is an essential part of water and

wastewater treatment Unit processes are designed for

spe-cific flow levels, and process adjustments (e.g.,

adjust-ments made on pumping rates, chlorination rates, filter

rates, aeration rates, etc.) are based upon current levels of

flow; in many cases, they are controlled by flow rate

adjustments Accurate flow measurement is a key element

in any attempt to identify, correct, and prevent operational problems Therefore, it is important to operators tasked with operating the plant at optimal efficiency

In this chapter, we briefly discuss methods of measur-ing flow, many of the calculations used to determine flow, and various flow measurement problems

9.2 METHODS OF MEASURING FLOW

Measuring the flow in water and wastewater operations requires a thorough, detailed understanding of the process and the substance being measured Two factors that deter-mine the method of flow measurement and the flowmeter most suited to an application are the quantity of the flow and the type of substance being measured

We already mentioned one of the methods of measur-ing flow, usmeasur-ing the Q = A ¥ V formula method When using this method, we measure the velocity of the flow and the channel width/water depth first, then the formula

is used to find flow rate This procedure can be used in any location where you can measure the water cross-sectional area and velocity

Note: The flow rate of a substance can be described using a number of terms including feet per sec-ond, gallons per minute, cubic feet per minute, and tons per hour The unit chosen to indicate flow rate is an important factor in flow measure-ment applications, and varies according to the indicating requirements specific to the process Another simple method that can be used to measure flow is known as the fill and draw method Accomplished

by measuring the amount of time required to transfer or pump a given volume of water from one point to another, use the fill and draw method at any location where changes

in liquid volume (or depth) can be measured

From the description of the two rudimentary measure-ment techniques just described, we can see that even in those cases where flow measurement is provided, some method can be found to measure — or at least estimate — flow rates However, the majority of water and wastewater operators do not measure flow by the two methods Instead, modern treatment plants and current practices normally include the use of other methods For example, the bucket and stopwatch technique has been replaced with other methods, up to and including complex electronic systems

9

284 Handbook of Water and Wastewater Treatment Plant Operations

9.2.1 W EIRS

The simplest, least expensive, and probably the most

com-mon type of primary measuring device used to measure

flow in open channels is the weir A weir is simply a dam,

rectangular obstruction, or V-notch crest (over which

water flows), placed in the channel so that the water backs

up behind it and then flows over it (see Figure 9.1-A and

Figure 9.1-B) For open channel measurements, weirs can

be used for rectangular and circular clarifiers (see

Figure 9.2) The crest or edge allows the water to spring

clear of the weir plate and to fall freely into the air Each

type of weir has an associated equation for determining

the flow rate through the weir The equation is based on

the depth of the liquid in the pool formed upstream from

the weir Simply, in measuring, measure the head a

spec-ified distance behind the weir (constriction) in the channel,

and then use the head to calculate the flow or to determine

the flow using a table or graph

The edge or surface over which the water passes is

called the crest of the weir, as shown in Figure 9.1-A

Generally, the top edge of the weir is beveled with a sharp

upstream corner (aptly called sharp-crested weirs) so that

the liquid does not contact any part of the weir structure

downstream, but rather springs past it This stream of

water leaving the weir crest is called the nappe A

disad-vantage of a weir is the relatively dead water space that

occurs just upstream of the weir where organic solids may

settle out, causing odors

Note: The operation of the weir is sensitive to any

foreign material or debris that may be present

upstream of the flowmeter or on the weir plate itself Therefore, the weir should be periodi-cally inspected and any accumulated debris removed This action will also reduce organic settling, and thus reduce odors

9.2.2 T HE O SCILLATING D ISK W ATER M ETER

Many individual household and apartment dwellers are familiar with the oscillating disk water meter, even if they

do not think they are They at least they know that there

is a meter that is read by the utility routinely and that their water bill is based on that meter reading

FIGURE 9.1 (A) Rectangular weir; (B) triangular V-notch weir (From Spellman, F.R and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

Weir crest

Head

Head

Weir crest

A.

B.

Rectangle weir

Triangular weir

Crest length

V-notch angle

FIGURE 9.2 (A) Weir overflow for rectangular clarifier; (B) weir overflow for circular clarifier (From Spellman, F.R and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

Flow Rate (gpd)

Weir

Flow Rate (gpd)

Weir A.

B.

Flow Measurement 285

This standard household water meter is known as the

oscillating disk water meter, which is a common

positive-displacement meter The meter has a measuring chamber

of known volume containing a disk that goes through a

cyclic motion as water passes through A recording register,

which can be mounted exterior to the house, records the

rotation resulting from filling and emptying of the chamber

This type of meter is very reliable, simple in construction,

highly sensitive, accurate, and has low maintenance costs

Note: For customers requiring high flow rates, another

type of water meter is used: the compound meter

The general-service compound meter consists of

a positive-displacement, current meter with an

automatic valve arrangement that directs water

to the current meter during high rates of flow and

to the displacement meter at low rates

9.2.3 F LUMES

Besides the weir, another device commonly used to measure

flow in open channels is the flume Figure 9.3 shows a

Parshall flume — the most commonly used measuring

device The Parshall flume is named after Dr Ralph L

Parshall formerly of the U.S Soil Conservation Service In

1922, Dr Parshall modified the existing venturi flume

design This perfected device measures the head a specified

distance behind the narrow point (throat of the flume), then

the head measurement is used to calculate the flow or the

flow is determined by using a table or graph Because

waste-water contains suspended and floating solids, it prohibits

the use of enclosed meters This is where the flume comes

in The principal advantages of the Parshall flume are its:

1 Capabilities for self-cleaning (i.e., its design

and smooth construction does not offer any

place where solids may collect behind the

metering device)

2 Relatively low head loss

3 Ability to function over a wide operating range

while requiring only a single head measurement

Again, these characteristics make it particularly suitable for flow measurement in wastewater operations The flume

is also used in water flow measurement applications

9.2.4 V ENTURI M ETER

A venturi is a restriction with a relatively long passage with smooth entry and exit The venturi meter is another flow measuring device found in pipe systems in waste-water collection systems and treatment plants It utilizes the principle of differential pressure — the flow must pass through a section with a smaller diameter in a device The change in pressure that occurs while passing through the smaller diameter section is related to the rate of flow through the pipe It is often used in wastewater streams since the smooth entry allows solids to be swept through instead of building up as it would in front of an orifice

9.2.5 M AGNETIC F LOWMETER

The magnetic meter (magmeter) is another flow measuring device commonly used to measure flow of the wastestream through pipes In operation, wastewater is passed between the poles of a magnet The flow creates an electrical current that the meter measures The amount of current produced

is related to the amount of flow

With obstructionless design, there are no moving parts

to wear and no pressure drop other than that offered by a section of pipe with equal length and inside diameter Moreover, the magmeter has the advantages of a linear output, corrosion-resistant wetted parts, and highly accu-rate output “The greatest disadvantage of this type of meter is its initial cost and the need for trained personnel

to handle routine operation and maintenance.”2 Table 9.1 provides a list of many different types of methods and devices applicable to fluid flow measurement

9.3 FLOW MEASUREMENT CALCULATIONS

While it is true that flow can be measured electronically

or by using various tables or charts, water and wastewater operators should be skilled in making flow computations Even with the use of a chart or graph, appropriate conver-sions and calculations are required

In this section, we discuss the calculations required to determine flow rates using the fill and draw, V-notch weir, and the Parshall flume We also provide a few simple flow calculation problems

9.3.1 C ALCULATION M ETHOD U SED FOR F ILL AND D RAW T ECHNIQUE

The mathematical procedure for determining flow in gal-lons/minute is:

FIGURE 9.3 Parshall flume (From Spellman, F.R and Drinan,

J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

Throat

Top view

Converging

outlet

Flow

Stilling well for

measuring hear

286 Handbook of Water and Wastewater Treatment Plant Operations

(9.1)

9.3.2 C ALCULATION M ETHOD U SED FOR

V ELOCITY /A REA T ECHNIQUE

For determining flow in cubic feet per second or gallons

per minute using the velocity/area technique, the

follow-ing equations can be used:

(9.2)

(9.3)

9.3.3 C ALCULATION M ETHOD U SED

FOR V-N OTCH W EIRS

Use a chart or graph and make appropriate conversions,

then use the following equation:

(9.4)

where

H = Head in feet

K = Constant related to the weir angle

9.3.4 W EIR O VERFLOW (W EIR L OADING R ATE )

Weir overflow rate is the effluent flow rate expressed in gallons per linear foot of weir per day (see Figure 9.2) It can be used to evaluate actual operating conditions by comparing current values with design specifications

(9.5)

TABLE 9.1

Types of Flow Measurement Devices

For Pressure Pipes

For Open Channels

a w = water application; ww = wastestream application

Source: Adapted from Qasim, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic

Publ., Lancaster, PA, 1994, p 221 With permission.

Flow Q gal min

Tank Volume ft 7.48 gal ft

min

( )

Re Time quired

Q ft3sec

( )¥ ( )

Channel Width ft Water Depth ft V ft sec Q

MGD

gal min

Q ft 1, 000, 000

3 3

¥

sec

sec 1

Q ft( 3 sec)= ¥K H2 5.

Weir Overflow

Weir

Q gal d Length ft

( )

Flow Measurement 287

Problem:

The circular settling tank is 90 ft in diameter and has a

weir along its circumference The effluent flow rate is 2.50

MGD What is the weir overflow rate in gallons per day

in feet?

Solution:

9.3.5 C ALCULATION M ETHOD FOR P ARSHALL F LUME

As with the V-notch weir, calculating flow through a

Parshall Flume requires the use of charts or graphs and

appropriate conversions Then, the following equations

may be used

1 For flume throats less than 12 in wide

(9.6)

where

K = Constant related to the throat width

n = exponent constant related to throat width

2 For flume throats 1 to 8 ft wide

(9.7)

where

W = Throat width

L = W0.026

9.3.6 T YPICAL F LOW M EASUREMENT

P RACTICE C ALCULATIONS

The answers to the examples provided in this section are

derived using equation standard Q = AV, including

con-versions where appropriate

Problem:

A grit channel 3 ft wide has water flowing to a depth of

16 in If the velocity through the channel is 0.8 ft/sec, what is the ft 3 /sec flow rate through the channel?

Solution:

Problem:

A grit channel 3 ft wide has water flowing at a velocity

of 1.3 ft/sec If the depth of water is 15 in., what is the gal/d flow rate through the channel?

Solution:

Problem:

A grit channel 35 in wide has water flowing to a depth

of 9 in If the velocity of the water is 0.80 fps, what is the ft 3 /sec flow in the channel?

Solution:

9.4 FLOW MEASUREMENT OPERATIONAL PROBLEMS

Operators are often responsible for troubleshooting flow measurement problems Our experience indicates that flow measurement problems (indicative of problems with the flow measuring method or device used) typically fall into two categories: (1) a sharp drop or increase in recorded flow, or (2) inconsistent or inaccurate flow mea-surement using a weir

A number of causes could be responsible for a sharp drop or increase in recorded flow For example, the prob-lem could be caused by an obstruction to the float (if used)

Removing the obstruction and/or keeping the float clean and free of grease correct this problem Another type of flow measuring device may malfunction because of improper airflow or a damaged bubbler tube In correcting this problem, the bubbler tube should be cleaned, the airflow adjusted, and grease removed from the assembly

3 in 0.9920

6 in 2.060

9 in 3.070

Weir Overflow .50 1, 000, 000 gal MG

ft 8846.4 gal d ft

¥

=

2

3 14 90

Q ft( 3 sec)= ¥K Hn

Q ft( 3 sec)= ¥4 W¥ H1 522 L

3 ft ¥ 1 3 ft ¥ 0 8 ft sec = 3 12 ft3 sec

3

2 9 ft ¥ 0 75 ft ¥ 0 80 ft sec = 1 74 ft3 sec

288 Handbook of Water and Wastewater Treatment Plant Operations

In another type of measuring device, the problem might

be indicative of grease buildup on magnetic meter coils

Solving this problem is a simple matter of removing

grease buildup A weir plate clogged with debris could

also cause a sharp drop in recorded flow To correct this

problem, debris should be removed and the frequency of

weir cleaning should be increased

Inconsistent or frequent inaccurate flow measurement

using a weir usually indicates that the weir is not level

and needs to be adjusted

9.5 CHAPTER REVIEW QUESTIONS

AND PROBLEMS

9.1 Why are flow measurements important?

9.2 A grit channel 2.5 ft wide has water flowing

to a depth of 18 in If the velocity of the water

is 0.8 ft/sec, what is the ft3/sec flow in the

channel?

9.3 A grit channel is 2.5 ft wide with water

flow-ing to a depth of 15 in If the flow velocity

through the channel is 1.6 ft/sec, what is the gal/min flow through the channel?

9.4 A grit channel 3 ft wide has water flowing to

a depth of 10 in If the velocity through the channel is 1 ft/sec, what is the ft3/sec flow rate through the channel?

9.5 If you had the choice of installing a weir sys-tem or a Parshall flume flow measurement device in your plant, which one would you choose or prefer? Why?

REFERENCES

1 Spitzer, D.W., in Flow Measurement: Practical

Guides for Measurement and Control, Spitzer,

D.W., Ed., Instrument Society of America, Research Triangle Park, NC, 1991, p 3.

2 Qasim, S.R., Wastewater Treatment Plants:

Plan-ning, Design, and Operation, Technomic Publ.,

Lan-caster, PA, 1994, p 226