# PRT 140: Lesson 4 Level

Contents

## 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

### Terms to Know

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

### Innage/Ullage

Question: Why might you use one or the other?

### Direct/Indirect Level Measurement

Instrument senses another property that is used to calculate level

### Direct/Indirect Examples

Direct Level Instruments

• Sight glass
• Float
• Dipstick
• Tape gauge

Indirect Level Instruments

• Bubbler
• dP cells
• Displacer

Interface

Meniscus

### 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?

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?

• 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

#### Direct Measuring Instruments

• Sight Glass — see examples
• Float
• Gauge Tape

### dP vs. PI

#### 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

• 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

• 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

### 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?

### 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