PRT 140: Lesson 4 Level

Objectives

  • Define ‘level’ and related terms
  • Describe common types of Level instrumentation
  • Demonstrate level measurements – bubbler, sight glass
  • Perform level calculations and conversions
  • Review Level instrumentation symbols on PID’s
  • Perform pressure, temperature, level calculations – review

Reading

Terms to Know

  • Innage
  • Outage/ullage
  • Direct level measurement
  • Indirect level measurement
  • Interface
  • Meniscus
  • Hydrostatic Head Pressure
  • Level instruments, per presentation

Innage/Ullage

Question: Why might you use one or the other?

A diagram of fluid in a tank
Innage or Ullage
[image 140-4-01]

Direct/Indirect Level Measurement

Instrument directly reads the level

 

A diagram of a sight glass used for direct measurement
Direct Measurement
[image 140-4-02]

Instrument senses another property that is used to calculate level

A diagram of a pressure gauge to depict indirect measurement
Indirect Measurement
[image 140-4-03]

Direct/Indirect Examples

Direct Level Instruments

  • Sight glass
  • Float
  • Dipstick
  • Tape gauge

Indirect Level Instruments

  • Bubbler
  • dP cells
  • Displacer

Interface / Meniscus

Interface

a diagram showing interface
Interface
[image 140-4-04]
Meniscus

An image of a miniscus viewed through a pipette
An example of a meniscus
[image 140-4-05]

Level – Hydrostatic Head Pressure

Pressure exerted by the depth of the liquid column (we’ve looked at this before…)

P = (hydrostatic head pressure) = in. H2O

h = height of the liquid column (liquid level) = in.

SG = specific gravity of the liquid (water SG = 1)

P = (1 in wc) x (h in. ) x (SG)
                     in.

Or P = h x SG (if everything is in the right units)

UNITS of Level vs. Pressure

Units of level = a length, height measurement: ft, in, mm, etc.

Units of hydrostatic head pressure = length, height of a particular fluid:

  • in. H2O, mm Hg, in. w.c.,
  • Sometimes in ‘feet of head’, implying water (this is used for pump data, ref. PRT 130)

Example 1

An open tank contains a liquid with a specific gravity of 1.735. If the level of the liquid is 123 in., how much head pressure, in in. H2O, will it exert?

A diagram illustrating h and PI for the example problem.
The open tank which contains a liquid, specific gravity 1.735
[image 140-4-07]
 

Equation to use:

  • P = (1 in. H2O)(h)(SG)       or P = h x SG
    in.
  • SG = 1.735
  • h = 123 in.
  • P = (1 in. H2O)(123 in.)(1.735)
    in.
  • P = 213.4 in. H2O

Level

If you know the hydrostatic head pressure and the SG, you can calculate level:

  • P = h x SG, so
  • h = P/SG
  • h is in.
  • P is in. H2O

IMPORTANT: Data must be in the right units for this simple equation to work. Always verify units before using equations.

Example 2

An open top tank is filled with a liquid that has a specific gravity of 0.873. The liquid exerts a head pressure of 193 in. H2O. What is the liquid level in inches?

 

A diagram illustrating h and PI for the example problem.
The open tank which contains a liquid, specific gravity 1.735
[image 140-4-07]
  • P = (1 in. H2O)(h)(SG) /in.
    • P = 193 in. H2O
    • SG = 0.873
  • 193 in. H2O = (1 in. H2O)(h)(0.873)/in.
  • h = (193 in.) / 0.873
  • h = 221.1 in.

Be very careful with units – this is not water, so giving a level height as in. H2O makes no sense at all!

Level Instruments to Know

  • Sight Glasses
  • Float
  • Tape Gauge
  • dP cell
  • Bubbler
  • Displacer
  • Ultrasonic, Radar, Nuclear
  • Load Cells

Direct Measuring Instruments

  • Sight Glass – see examples
  • Float
  • Gauge Tape
Direct measurement example
An example of a direct measure instrument.
[image 140-4-06]

dP vs. PI

A diagram depicting how dP differs from PI
dP vs. PI
[image 140-5-06]

PI Measures All Pressures

  • PI measures PX (vessel P) + PH (hydrostatic)
  • P = PX + PH
  • PX = 5 psi
  • PH= h x SG = 67 in. x 1 = 67 in. H2O
    • 67 in. H2O = 2.42 psi – (how do we know this?)
  • P = 5 psi + 2.42 psi
  • P = 7.42 psi at the PI

dP Cell – Measures Level Regardless of Vessel Pressure

  • Low side of dP measures PX, vessel pressure
  • High side of dP measures PX + PH (hydrostatic)
  • Since it’s a differential, the final reading is
  • dP = (PX + PH) – PX
  • dP = PH
  • PH = h x SG
  • PH= h x SG = 67 in. x 1 = 67 in. H2O
    • 67 in. H2O = 2.42 psi
  • dP = 2.42 psi

Advantage of dP Cell?

  • dP cell measures only the hydrostatic head from the liquid level – the vessel/system pressures are canceled out.
  • To use a single PI, you have to KNOW the vessel pressure and include that in any calculations.
  • Consider how the reading on the PI will change with vessel pressure – how do you know the level is changing or not?

Level – Bubbler

A diagram of a bubbler with labeled components
A diagram of a bubbler
[image 140-4-08]
  • What is the pressure at the bottom of the dip tube?
  • What is the formula for the pressure at the bottom of the liquid level, h?
  • P = h x SG

Example Question

What is the water height in inches, h, if the pressure on the bubbler reads:

  • 1.5 psi
  • 13 psi
  • 72 psi

0.433 psi = 1 ft water     (WHERE DOES THIS COME FROM?)

0.433 psi = 12 in. water     (because 1 ft = 12 in.)

1 psi = 12 in. water/0.433

1 psi = 27.7 in. water

Bubbler       P = h x SG, so P/SG = h

FIRST – convert all pressures to in. water

  1. P = 1.5 psi = 41.6 in. H2O
  2. P = 13 psi = 360.1 in. H2O
  3. P = 72 psi = 1994.4 in. H2O
    • h = P/SG     (SG =1.3)
  4. h = 41.6/1.3 = 32 in.
  5. h = 360.1 / 1.3 = 277 in.
  6. h = 1994.4 / 1.3 = 1534.15 in.

(NOTE that the level units are different from P units)

How to Recognize Level Instruments in the Field

Many dP and P instruments are used to indirectly measure level. How do you identify them in the field?

  • Look at how they are connected/configured in the process.
  • Single PT – would have to be at the bottom of a liquid column, right? (not too commonly used this way)
  • dP is very common
  • First – make sure it’s a dP transmitter – H/L connections, possibly two visible leads to process
    • One at bottom of a liquid column, one to atmosphere
    • One at bottom of a vessel, one in the vapor space of a vessel
    • Two connections to a liquid-full container, at a specific height differential
    • One to a bubbler line, one to atmosphere
    • Probably many more

The bottom line is – look at how it’s configured, think what information you get from that data

Displacers

A diagram of a displacer
A displacer
[image 14-4-09]
A diagram illustrating buoyancy measured in pounds
Buoyancy reading
[image 140-4-10]

Other

Ultrasonic/Radar: Measure ullage distance, infers level. Used when physical sensors may not work (i.e. asphalt tankage)

Nuclear: uses gamma radiation to detect matter inside the vessel. Used for difficult measuring situations, extreme conditions

Load Cells: Weigh the contents, can calculate level

  • What other info do you need to make this calculation?

Level – PID Symbols

An example diagram with PIDs
PID symbols
[image 140-4-11]

Review

In a closed container,

P1V1/T1 = P2V2/T2

Pressure and Temperature must be in ABSOLUTE UNITS –

  • P = psia
  • T = K or R

Volume units just have to be consistent

Step 1 = get all units and variables identified

450 psig, 125 C, 437 ft3                      =                    235 psig, 564ft3, ???C

Variables Data, units
P1 450 psig
T1 125 C
V1 437 ft3
P2 235 psig
T2 ?
V2 564 ft3

Step 2 = get all units correct (absolute T, P)

Variables Data, units Data, in absolute units
P1 450 psig 464.7 psia
T1 125 C 398 K
V1 437 ft3 437 ft3
P2 235 psig 249.7 psia
T2 ? ?
V2 564 ft3  564 ft3

Step 3, set up and solve equations

(464.7 psia x 437 ft3) / 398 K = (249.7 psia x 564 ft3) / T2

T2 = (249.7 psia x 564 ft3 x 398 K) / (464.7 psia x 437 ft3)

T2 = 276 K

T2 = 3 C (answer should be in same units as original data, unless specified otherwise)

Scaling Calculations

Level instrument operating range is
5 ft – 85 ft. What is the 4-20 mA reading at different levels?

VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

Level:  LRV = 5 ft  URV = 85 ft       Span = 80 ft

Signal:  LRV = 4 mA  URV = 20 mA   Span = 16 mA

So:

?? mA = {[(Level Reading – 5 ft)/80 ft] x 16 mA} + 4 mA