Need of Stilling Well (SW) – as per API
Introduction
A SW is a ‘should’ – and highly
Field
recommended, as most tank roofs are
instrumentation not stable enough.
System A SW is the only possibility for
millimeter accurate measurement in
Case Studies floating roof tanks.
Only for smaller tanks (<5000 bbls.)
the recommendation is dropped.
Also the Dutch NMI and German PTB
don’t see it as a ‘must’ – but the tank
roof needs to be ‘enough stable’ –
which is hardly the case – you have to
prove the roof is stable enough
(influence < 2-3mm!)
Slide 1
Still pipe design according API
Introduction
Field
instrumentation
System
Case Studies
Slide 2
What our recommendations for SW for Radar are:
Introduction Preferable two rows – pitch 300mm (depends on viscosity of product)
Holes staggered on both sides, in a straight vertical line, de-burred – preferably
300mm
Field conically drilled.
instrumentation
SW diameter:
System 6” for normal hydrocarbons in Fixed Roof Tank
8” for more viscous products and medium size FRT’s (<50m)
Case Studies 10” for larger FRT’s (when SW is also used anti rotational device’ for the FR)
2” for Bullets and horizontal cylindrical tanks
Hole diameter:
1/10 of pipe diameter)
Highly recommended for new installations:
Material: SS
Separate SW for manual gauging and sampling (4”)
15 cm
Separate ‘Open’ (perforated) thermo well for temperature measurement
Thermo well open ended and above ‘water sump’ (if applicable)
30 cm
End of SW: below zero point (datum plate) and 300mm above bottom so that a reflector
plate can be placed below.
Slide 3
Nominal SW diameter
Introduction
‘Recommended’ and not
Field ‘must or ‘should’!
instrumentation
System
Case Studies
Realize that this Chapter only contains details for ‘non pressurized tanks’. (All
light hydrocarbons having a Reid vapor pressure above 100 kPa (15 psi) and
underground storage tanks are excluded)
Hence different (smaller!) diameter SW’s are not per se ‘non API
compliant’
Slide 4
Stilling well requirements
Introduction
Field
At any transition (e.g. when using a ball
instrumentation
valve or mending pipe segments), no gap
System may be created exceeding 1 mm.
Case Studies The stilling well must be smooth on
the inside (average roughness Rz ≤ 6.3
μm). Use extruded or parallel welded
stainless steel pipe. An extension of the pipe
is possible with welded flanges or pipe
sleeves. Flange and pipe have to be properly
aligned at the inside.
Shifted pipes should not be welded
Stilling well must be open at the bottom.
Slots offset 180° (not 90°).
Slide 5
Stilling Well design aspects
Introduction
Field
instrumentation Design aspects for SW:
System
•Holes instead of slots
Case Studies •SW fixed to the bottom
•SW freely moveable on the top (no influence from roof
movement)
•Not heavily corroded
•Not bended – nearly Straight
•Not too much adhering product
•Not ‘over-slotted’
•Minimization of tank bottom reflections
Slide 6
Fixed Stilling Well position (gauge reference height)
Introduction
Bottom area Top area
Field
instrumentation Side view Top view Side view Top view
System
Case Studies
45°
SW welded (fixed to bottom), outlet SW not welded to the roof, roof
with reflector (alignment 40°), movement does not influence the pipe
reflector away from tank wall, reflector position
plate used for Radar installations
Slide 7
Standard Stilling Well Drawings by Endress+Hauser
Introduction
Field
instrumentation
System
Case Studies
Slide 8
Stilling wells not acceptable for radar
Introduction
Field
instrumentation
System
Case Studies
Slide 9
Stilling wells not acceptable for radar
Introduction
Field
instrumentation
System
Case Studies
Slide 10
Stilling wells not acceptable for radar
Introduction
Field
instrumentation
System
Case Studies
Slide 11
Stilling wells not acceptable for radar
Introduction
Field
instrumentation
System
Case Studies
Slide 12
Stilling wells not acceptable for radar
Introduction
Field
instrumentation
System
Case Studies
Slide 13
6” UNI
Flange
Introduction
Field
Flexible
instrumentation
Rubber boot 6” flange
System
Bolts for
Case Studies adjusting SW
8” Roof Tank Roof
Nozzle
6” Stilling
Well
Slide 14
6” UNI
Flange
Introduction
Field
instrumentation
System
Case Studies
Slide 15
