.acc controller
.boiler acc
Description:
Initially, MV=SV, PV1=C.SP1, PV2=CSP2, a ≈ b, c ≈ d.
If steam demand ↑, MV↓
O/P of R/A Master Controller ↑, c↑ , a↑
c > d, d passes through LSS,
so no change in CSP1,
But a > b, ‘a’ passes through HSS,
CSP2↑, PV2↓ wrt CSP2
O/P of D/A Air flow controller ↓
ATC air damper opens more,
Combustion Air flow ↑, d↑,
d passes through LSS since d < c,
CSP1 ↑, PV1↓ wrt CSP1,
O/P of R/A Oil flow Controller ↑,
ATO oil control valve opens more,
Steam pressure restored to SV within a few cycles due to I-action of the Master controller.
The control scheme above ensures that air is increased ahead of fuel to prevent fuel-rich condition when load ↑. And similarly, fuel will be decreased ahead of air to prevent fuel-rich condition when load ↓
DP cell
Purpose and Overview
The boiler DP cell is used for remote indication of boiler water level, which is crucial for safe and efficient boiler operation.
Key Components
Main parts of the system:
- Boiler Drum: Contains water and steam under pressure
- DP Cell: Measures differential pressure between two points
- Equalizing Leg: Minimizes damping action and stabilizes measurements
- Valves: Used for system isolation and maintenance (NO and NC)
Operating Principle
DP cell working:
- It measures the pressure difference between the low pressure (LP) and high pressure (HP) sides of the boiler
- LP side connects to the bottom of the drum, measuring water column pressure
- HP side connects to the top, measuring water column plus steam pressure.
Key Equations
- LP = ρg(H1 + H2)
- HP = ρg(H1 + H2 + H3)
- ΔP = HP – LP = ρgH3
Where ρ is water density, g is gravitational acceleration, and H3 is the water column height above the lower tap.
Interpretation and Calibration
- ΔP is proportional to H3, indicating water level above the lower tap
- As water level (H2) decreases, H3 and ΔP increase
- The system is calibrated with 20 PSI for low level and 3-15 PSI for the operating range.
Practical Implications
- Important for remote monitoring
- It contributes to boiler safety and efficiency
- Water level changes according to DP cell output.
Air to close:
Air to close valves is normally held opened by the spring and require air pressure (a control signal) to close them – they close progressively as the air pressure increases.
Why ATC?
To provide failsafe arrangement.
Example:
Jacket water temperature controller. During failure air will shut, bypass valve open, controller will direct water to cooler. Excessive cooling may occur, but this is safe for engine.
Air to open valve:
Opening valves is normally held closed by the spring and require air pressure (a control signal) to open them – they open progressively as the air pressure increases.
Why ATO?
It is decided according to fail safe arrangement.
Example: A fuel supply valve, any failure of power or air supply will cause the valve to close by spring pressure.
Capacitance type level sensor:
.level sensor .capacitance type level sensor
Principle
A capacitive probe is placed inside the content of the tank where level is to be measured. This method is based on the fact that the capacitance between a stationary probe and the vessel wall depends on the liquid level around the probe and the fluid dielectric constant.
Principal Features
An insulated rod is inserted into the tank. Normally, a metal rod coated with PVC or PTFE is used to prevent corrosion.
The capacitance between the rod and the tank walls has two components: C1 above the liquid (or solid) surface and C2 below. These capacitances depend on the geometry of the installation (for example. rod diameter and the distance to the wall) and the dielectric constants of the liquid and the vapour above the surface. The basic capacitor equation is:
Here, C=capacitance
As the liquid level rises, C1 will decrease and C2 will increase in value. The two capacitors are effectively in parallel, and as liquids and solids have a higher dielectric constant than vapours, the net result is an increase in capacitance with level.
The change in capacitance is small that abridge/amplifier circuit is used.
A whitstone bridge is used to sense the change in level. The whitstone bridge stays at a balanced position, no current flows through its output terminals. When the capacitance changes due to the rise of liquid whitstone bridge will become unbalanced and its output terminal will conduct electricity. This output signal is then used as a process value.
The main benefit of capacitance probes is that it works with a wide range of liquids and solids.
These type of level sensor have to be designed specifically for each and every application with regard to tank geometry and dielectric constant of the liquid present in the tank.
