Amendment 0003 QnA memo.pdf
PDF 276 KB Posted
- Attached to
- Multi-turn valve actuator, stem, and pedestal
- Solicitation number
- W912ES26QA004
About this file
This is a Memorandum for Record providing an official response to a bidder question for solicitation W912ES26QA004 regarding Break-to-Open (BTO) torque requirements for a multi-turn actuator sizing application.
The required unseating (break-to-open) torque for the existing gate is 156 ft-lbs, calculated using the stem factor formula from AWWA C560-21 with an unseating working thrust of 8,200 lbs and a stem factor of 0.019 feet. The calculations are based on ACME thread specifications (2.000-0.25P-0.50L-ACME-2G-LH) with a basic pitch diameter of 1.875 inches, thread lead of 0.500 threads/inch, coefficient of friction of 0.15, and a half-thread angle of 14.5 degrees. Per specification Section 35 20 16.54, the actuator must provide a minimum 20 percent reserve capacity, which establishes a required torque rating of 187 ft-lbs. Bidders are directed to consult the specifications and plan sheet M-001 for additional requirements related to actuator selection and performance.
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MEMORANDUM FOR RECORD
Subject: Response to bidder question for solicitation W912ES26QA004
Bidder question:
What is the required worst-case Break-to-Open (BTO) torque for the existing gate, in lb-ft or another applicable torque unit, to be used for sizing the multi-turn actuator?
BLUF: The unseating (break-to-open) torque is 156 ft-lbs. Note: the specifications state that the actuator must have a minimum 20% reserve capacity, which is 187 ft-lbs. See specifications and plan sheet for additional requirements.
The following calculation is based on information given in the specifications, plan sheet, and references.
Unseating (break-to-open) torque is found by multiplying the unseating thrust by the stem factor. Paragraph 1.5.1 of specification Section 35 20 16.54 reads:
"The effect of the stem factor in selecting the motor/actuator assembly must be calculated using the stem factor formula in AWWA C560 with a minimum coefficient of static friction for lubricated surfaces of O. 15."
The stem factor formula is found in AWWA C560-21 on page 12:
"Stem factor (feet) = (dm/24) x ((L + rrµdm x sec a) I (rrdm - µL x sec a)), where dm = basic pitch diameter (inch), L = thread lead (thread/inch), µ = coefficient of friction
(e.g., 0. 15 or 0.20), a = ½ thread angle (e.g., 14 ½ degrees for 29 degrees Acme).
Note: Calculation of basic pitch diameter varies by thread type."
The ASME B1 .5 thread designation for ACME threads is given in plan sheet M-001 as:
2.000-0.25P-0.50L-ACM E-2G-LH
ASME B1 .5 gives the basic pitch diameter and thread angle. From everything that's given, the stem factor is:
AWWA C560-21 stem factor basic pitch diameter dm 1.875 inch lead L 0.500 threads/inch pi n: 3.142 coefficient of friction µ 0.150
1/2 thread angle (degrees) a 14.500 degrees
1/2 thread angle (radians) a 0.253 radians secant (a) sec a 1.033 stem factor 0.019 feet
The unseating torque is given below. Please also note the reserve capacity requirement from Section 35 20 16.54 paragraph 1.5.1:
"The actuator must be able to produce the working load plus an additional 20 percent reserve capacity."
With the stem factor of 0.019, the unseating (break-to-open) torque and reserve capacity torque are:
Unseating torque requirements
Unseating (working) thrust Fworking 8200 lbs
Unseating (break-to-open) torque 156 ft-lbs
Unseating torque, 20% reserve capacity 187 ft-lbs
File details come from the government source that posted it. Updated .